Let's start with this subject, because it's kind of important~ I won't cover every single difference, but I'll hit the high points~ (For shorthand I will refer to Civilization 4 as CIV, and Civilization 5 as CiV).

1. One unit per tile. This is the most major and possibly the most controversial change in CiV. Anyone who's play past Civ games starting with the very first Civilization knows about unit stacking, which is no longer possible in CiV (except in some limited fashion)~ Gone are the days of the "stack of doom," where you combine all of your units into one big stack that is not particularly vulnerable to any type of unit~ For better or for worse, no more than one combat unit can share a tile. This, combined with the new hexagonal grid (as opposed to the square grid of past Civs), places a heavy emphasis on positioning~ You receive bonuses for "flanking" units, or surrounding them with several of your troops~ This can also lead to "choke points" and a very slow advance of armies across the map, depending on the terrain~ You need to think about how to use this to your advantage~ As mentioned previously, units can still stack in a limited fashion--non-combat units such as workers and settlers can stack in the same tile with a combat unit (though two non-combat units cannot stack, which I regard as a questionable design decision)~ Because of this change, there are also now "ranged" units, which can attack from two or more tiles away from their target, without fear of suffering damage in the process~ Typically such units are very vulnerable to attack and should be protected by melee units~

2. Cities Defend Themselves. No more walking a warrior into an empty capital city! All cities are considered viable combatants in and of themselves, and can be further garrisoned by a combat unit to increase their strength~ Cities are not only tough and take several attacks to take--which damages the attacking unit in the process--but they also possess a ranged attack which can be leveraged against approaching units~ Stationing a ranged unit in the city will allow the ranged unit to also make ranged attacks against attacking units without placing it in danger, making them ideal for city defense~ It is typically accepted that unless you have overwhelming numbers, even an undefended city will require siege to capture, especially if it has defensive structures such as Walls or a Castle~ It should be noted that when you attack a city, you always are attacking the city itself, not any units stationed in the city, but if the city falls, any units inside will be destroyed (or captured in the case of Workers or Settlers)~

3. Combat is very different. Combat in CiV is not necessarily lethal for the attacking or defending unit, another major change from CIV and past Civ titles, in which case combat would almost always result in the death of the attacker or defender~ In CiV, when combat takes place, the units damage each other but neither is guaranteed to die, and they usually do not, unless there is an overwhelming tech superiority on one side, or one of the units is already damaged~ It often takes several combats to wear down a unit to the point where it is destroyed, which combined with one unit per tile, makes positioning extremely important for quickly focus-firing and eliminating enemy units, lest they escape, heal up, and pester you once again~

4. Commerce is gone. Another questionable decision from my point of view, I nevertheless accept it as part of CiV~ In past games, the "commerce slider" that converted all commerce earned by your cities into either gold or science (or in some cases, happiness, culture, or espionage) was very important, and with the exception of some outlying strategies, balancing commerce was essential to maximizing the output of science in your empire~ This is gone in CiV~! Instead, science is based mostly on population, and most things that produced commerce in the past now directly produce gold~ There are exceptions, of course, certain buildings produce either gold or science, as well as some other things such as social policies and trades with other civilizations, but the point is that commerce is GONE, non-existent as a concept in CiV~

5. Global yields exists in edition to Local. Well, this is something that is rarely mentioned and often overlooked, but I consider it a very important distinction from past games and CIV in particular~ In Civilization 4, pretty much all yields were local--produced directly by your cities! Whether it was science, gold, food, culture, espionage, happiness, health, whatever, everything was produced in a city before it was added to your empire's total output~ Income from trade routes, commerce output based on the slider, you name it, it was produced in a city~ This had several important implications, notably the presence of modifier buildings on a city-by-city basis was extremely relevant~ In CiV, MOST yields are still produced locally in cities, but there are also "global" yields~ For example, the gold income from trade routes is no longer centered in any city, it is a global yield. This means that it is not subject to gold modifiers from Markets and Banks (both of which increase local gold output by 25%), even if you have such a building in every single city~! You may also get some yields of gold, science, culture, and other such types from various social policies, city-states, and religious beliefs that are global rather than local~ For example, the pantheon "Messenger of the Gods" produces "2 science in cities with a trade route," which is a local yield, but the "Tithe" founder belief produces 1 gold for every 4 followers of the religion as a global yield--it simply totals up all followers of the religion in the world and adds 1 gold for every 4 to your total, without modifying it at any local level~ This is perhaps most noticeable with Happiness, the new "limiting mechanic" on expansion~ Happiness is produced both globally and locally, but all that matters is your global total--your empire is either happy or unhappy (with several levels of unhappiness, each penalizing you more), it is not determined on a city by city basis~!

6. Social policies replace Civics (and governments from older civ games). The new "government" system of CiV is called the social policy system, and is based primarily on culture~ Each time you reach a certain amount of global culture in your empire, you can adopt a new social policy, which provides certain benefits to your Civilization~ Social policies are divided into ten separate "trees," with branching paths--you must adopt certain social policies to gain access to new ones~ I intend to go more in-depth on social policies and strategies in using them later on, but what's important to know here is that the cost of social policies increases as you gain more, and as you gain new cities~ Certain social policy trees are generally tailored to different play-styles, and emphasize different aspects of the game, such as science, gold, combat, expansion, etc~ In addition, social policies tie into the newly designed Cultural Victory condition, as you must fully fill out five social policy trees before you may build the Utopia Project, the completion of which earns you a cultural victory~

7. Civilization 5 is easier than Civilization 4. I honestly don't think I have a problem with stating this as objective fact~ Civilization 5 is less complex than past games and therefore easier to learn~ Now, CiV was plagued by a lot of problems on release. Certain strategies were effective that were boring and unintuitive, while the "intended way to play the game" was even more boring and didn't involve many interesting decisions! This, combined with the fact that CiV is "easier" made the game get a bad reputation.

It has taken a lot of patches and an expansion, but I feel like enough things have been fixed to make it a fun game now~ There are certainly more interesting decisions to make, and several concepts that I felt were flawed have since been fixed~ Let me list a few of them here to just get a gist of things:

(a) In the past, the earlier, cheaper buildings typically had better yields than the more advanced buildings AND used to cost less in maintenance. For example, the Colosseum building provided FOUR happiness starting with the Classical era tech Construction and only 1 gold per turn in maintenance, but the Stadium, a Modern era building, would provide only two happiness and cost a whopping FIVE gold per turn in maintenance! Not only that, but the Stadium was more EXPENSIVE than the Colosseum in terms of production, meaning it took longer to build, and required you to already have a Colosseum and Theater in the city to build it! Who thought this was a good idea?! This is one of the reasons the "infinite city sprawl" strategy was so effective--it was far better to settle lots of smaller cities with the low-tech, cheap buildings than it was to build up massive cities with lots of infrastructure, because the smaller cities would cost less gold in maintenance and produce more output with less time required to develop! As long as you could deal with unhappiness generated by mass expansion (an easy task with the Meritocracy social policy and the old Forbidden Palace wonder) and control the growth of your cities so they did not cause you excess unhappiness, city-spam with a disregard for terrain was definitely the way to go. This has been fixed in that now earlier buildings are typically weaker than later era buildings, and in addition, the maintenance cost of later-era buildings has generally been lowered across the board~

(b) Research "slingshots" were exceedingly powerful in the older versions of CiV. This is primarily because Research Agreements and Great Scientists would grant you a free technology, regardless of the cost of the actual technology~ So naturally, it was better to manipulate your tech discoveries, while saving up Great Scientists and timing research agreements to suddenly make huge bounds down the tree, either to break into a new era (City-State bonuses increase as you enter new technological eras!) or secure a vital technology far down the tree! Now, Research Agreements and Great Scientists provide you with large amounts of science that vary based on your research rate, so while they still CAN provide you with an instant technology, it is not guaranteed, and the tech tree has been redesigned to control "slingshot" strategies~ There are still certain social policies and wonders that can provide free technologies, so "slingshots" still exist, but they are not as dominant or powerful as they were before~

(c) City tile yields. Oh, dear. You will notice instantly in CiV that the average tile yield of improved resources is lower than that of CIV. This isn't a big problem in and of itself, but it DOES place less emphasis on finding the perfect city site, which unfortunately can lead to all cities feeling the same~ City specialization was extremely vital in Civilization 4, as several production-based resources would encourage a city devoted to producing military, while commerce-based resources will give the city a science/gold feel~ Because of the lower tile yields in CiV, the difference between a resource and a regular tile was a lot smaller, and often the difference of one or two points of a particular yield. Unless you had a lot of great tiles, it simply did not add up~ This, combined with the old versions of Maritime city-states, which provided huge food bonuses to every city in your empire when allied, lead to city homogenization. Another problem was Trading Posts, which added 2 gold to a tile and came as early as the trapping technology. As Trading Posts were the only normal improvement that improved a tile's yield by 2 rather than 1, simply spamming trading posts was regarded as an optimal strategy, using the gold to buy units and structures rather than trying to produce them~ This problem has been attacked on several levels--firstly, buildings have been re-designed to provide additional benefits to certain resources near the city, which brings back city specialization. For example, the Stable building adds +1 production to every horse, cattle, or sheep resource worked by the city, while the Granary adds +1 food to every wheat, deer, or banana resource worked by the city~ This leads to bringing back city specialization, in that you will build different buildings in a city based on what terrain is nearby, and the overall yields will be different enough to focus the city on different things~ In addition, tile yields now increase more with later technologies--previously, only farms benefited from this, gaining +1 food when next to water with Civil Service, and +1 food otherwise with Fertilizer. Now, all tile improvements improve with later technologies (Mines and Quarries with Dynamite, Plantations and Pastures with Fertilizer, Lumber-mills with Scientific Theory, etc), and the Trading Post has been brought to heel as well, coming with a later technology (Guilds rather than Trapping), and only increasing the gold yield of the tile by 1 until Economics~ Note that certain social policies and religious beliefs may also adjust or add to tile yields~ In addition, the power of Maritime city-states has been significantly reduced, now providing no benefit to non-capital cities unless you are allies, and the amount of food provided to non-capitals has been reduce significantly (I don't believe it ever goes over 1), so it is no longer practical to feed your empire purely on alliances with Maritime city-states~

More to come later, I hope~

One of the new concepts introduced in Civilization 5 is social policies. This is one of the best additions to the game, in my opinion, as it offers a very wide variety of choices that definitely influence how you play the game~ The civics system of Civilization 4 worked well, much better than the monolithic governments of previous installments, but the social policy system is quite interesting and diverse, and the tie-in to culture is nice as it gives culture another in-game function~ It encourages you to actively seek culture for purposes other than a cultural victory and quick border pops~

So, basically, how it works is this: every time you reach a certain cultural threshold, you can select a new social policy. The amount of culture needed for the next policy grows exponentially as you gain more social policies, and in addition, every city you found also increases the cost of additional social policies by a % based on the map size~ This is another way CiV offers trade-offs to expansion, with increasing happiness risks also comes increased social policy costs, to make up for the fact that you'll have more cities generating more culture.

Three social policy trees are available from the start, with additional policy trees becoming available unlocking as you progress in technological eras. Furthermore, certain social policy trees are mutually exclusive--for example, the three Industrial era social policy trees are all mutually exclusive in that only one of the trees can be active at a time~ Though you can actually adopt social policies in multiple restricted trees, only one of the two trees will be active at a time, and a period of anarchy will be required to switch between them. I would not generally recommend taking policies in opposing trees~ Finally, all social policies have an "opener" policy that is required to begin taking any policies in the tree--that is, you must expend one policy to "adopt" a policy tree. So while there are only five icons in each tree, you must actually expend six policies to max them out. However, not only does adopting a policy tree provide a benefit, but you gain an additional (usually very potent) "free" benefit for adopting every policy in the tree!

Let us examine the social policies in depth now.

Tradition
The tradition social policy tree emphasizes growth, and its benefits are tailored towards smaller empires. Some of the benefits only affect your capital city, others give your entire empire bonuses based on the size of your capital, and some policies provide benefits for your first four cities. Generally, tradition is favored by "tall" empires--which are defined by empires that run a fewer amount of cities and do not expand much, but grow their cities to large populations and fully flesh out their infrastructure.

Tradition Policy Tree
Adopting Tradition increases culture in your capital city by 3, and greatly enhances the rate of acquisition of new tiles culturally throughout your empire. This bonus helps you continue to adopt new policies at a reasonable rate, while keeping you getting new tiles to work, which works well with the extra food the rest of the tree will give you~ Since the cultural bonus only applies to your capital city, it does not scale well with a massive empire.

Once all policies in the tradition tree are adopted, you are provided with a free Aqueduct building (saves 40% of the food box when the city increases in size) and +15% growth in your first four cities. This is a very strong benefit closely tied to growing big cities, but again you will notice it only affects a limited number of cities~ Now for the policies themselves~

Aristocracy: +15% production of wonders and +1 Happiness for every 10 citizens in a city. The second part of this policy's benefit for large cities is obvious, but the first part is not so obvious~ I would suppose that the idea is that a smaller empire is more likely to build wonders, since it can do so faster and it runs out of new things to build more quickly~ Choose your wonders carefully, and if the enemy adopts this policy, note that you may find wonder competition with them fierce~

Legalism: Free cultural building in your first four cities. Early on, this gives you a free monument in your first four cities~ While you can save it and actually get more potent cultural buildings if you adopt this policy later, once you already have monuments and/or amphitheaters and such, I wonder of that benefit, since getting the monuments early and devoting your production to other things may result in more culture in the long run~ Plus, legalism is required for the bottom two policies in the Tradition tree~ Note again this benefit does not scale past four cities.

Oligarchy: Garrisoned cities cost no maintenance and each garrisoned city has +50% ranged combat strength. This is just useful all-around, but saving on unit maintenance is always nice, and this makes your cities with garrisons even harder to take~ As you have fewer cities, it's nice to make sure you won't lose them~

Landed Elite: Requires Legalism. +2 food and +10% growth in the capital city. Simple, easy, this buffs growth in your capital city. Combines nicely with...

Monarchy: Requires Legalism. +1 Gold and -1 Unhappiness for every 2 Citizens in the Capital City. So your capital grows at an increased rate, but its growth provides less unhappiness than any other city, and you get free gold on top of that~ Very nice with the other policies in this tree, working together to form a cohesive whole~

Liberty
This policy tree favors rapid expansion. You could consider it the opposite of Tradition, and while it's not terrible to take both, generally they are tailored towards different strategies. The result of rapid expansion is more cities, and Liberty also provides a few benefits to each city in your empire--in other words, they become more powerful as you settle more cities. Which is the opposite of Tradition~

Liberty Policy Tree
Adopting Liberty grants you +1 culture per city. Simple and effective, this gives you an increasing amount of culture for every city you settle, ensuring you will always be expanding borders and adding culture as you continue expanding. Though keep in mind that +1 culture per city does not count much in the face of the expanding cultural costs of policies via expansion!

Adopting every policy in the Liberty tree gives you a free Great Person of your choice in your capital city. There is no way this is possibly bad~ Engineer to rush a wonder or manufactury, prophet to found a religion, scientist for an academy... do whatever you want, it's a solid benefit~

Republic: +1 Production per city and +5% Production when producing buildings. This is a generally useful benefit that, again, scales with the number of cities you have. Every new city benefits equally from this, so the best way to use it is to found lots of cities~ Oh, and you build a little faster~

Citizenship: Tile Improvement rate increased by 25% and a free Worker appears outside the capital. Okay, free worker is never, ever bad. Plus, you make improvements faster! This is clearly aimed at keeping you on top of your expansion, as you constantly claim new land, you need to make sure your land is worth owning~

Collective Rule: Requires Republic. Production of settlers increased by 50% and a free Settler appears outside the capital city. Expand, build more settlers to found more cities, and keep expanding. That's what liberty promotes and this helps you in that regard very simply. And hey, you get a free Settler out of the deal too~

Representation: Requires Citizenship. Each new city you found increases the cost of future social policies by 33% less than normal, and begins a Golden Age in your civilization. So you wanna make a lot of cities? This policy is wonderful for you, as you can handle the increased social policy costs easier. Oh, and you get a free golden age! Who doesn't love Golden Ages~?

Meritocracy: Requires Citizenship. +1 Happiness in each city with a trade connection to the capital and -5% Unhappiness from Population in non-occupied cities. Another cost of more cities is extra unhappiness, which scales both with population and the number of cities in your empire. This directly counteracts unhappiness by the number of cities (so long as you connect each one to your trade network, which you should do anyway), and gives a small reduction in population-based unhappiness. Again, this works best with lots of cities~

Honor
The honor tree is for warmongers, no doubt there. It has a definite military focus, both offensively and defensively, but do not disregard the economic benefits either! It has some of those as well, as I will explain~

Honor Policy Tree
Adopting the Honor tree gives you culture for every barbarian unit you kill, gives your units a combat bonus against barbarian units, and notifies you of the location of new barbarian camps appearing in any previously explored territory. This is potent early on, but since the culture scales with the strength of the barbarian unit, you can get quite a bit of culture for killing barbarians even later in the game! Further, you will always know where the camps are, and your units are more capable in fighting barbarians.

Adopting every policy in the Honor tree grants you bonus gold for every ENEMY unit killed. If you are fighting any reasonable amount of war, this adds up stupidly fast. Very, very potent. Watch out for warmongers with maxed honor.

Warrior Code: +15% Production when producing Melee Military Units, and a free Great General appears outside your capital city. Free great general is nice, and you make front-line units faster. Simply and effective, and naturally, the more melee units you build, the more you get out of this policy.

Discipline: +15% Combat Strength for melee units that have a friendly melee unit in an adjacent tile. Your units are stronger in groups, and it's a good idea to group your units anyway for flanking bonuses--this just makes them EVER STRONGER.

Military Tradition: Requires Warrior Code. +50% Experience granted by combat. Look, if you're going to war, I consider this the most important policy you can adopt. There are very powerful promotions that unlock when your units reach high levels of experience, and this helps you get to them faster. A lot faster. You want Logistics on your Battleships, Machine Guns, and Bombers sooner, not later.

Military Caste: Requires Discipline. Each city with a garrison gains +1 Happiness and +2 Culture. Not a huge bonus, but nice for keeping your cities happy, and extra culture is always nice for more policies.

Professional Army: Requires Military Caste. Cost of upgrading military units reduced by 33% and +1 Local City Happiness from every Walls, Castle, Arsenal, and Military Base. Not only do you upgrade units at a cheaper cost, but you also gain free happiness from defensive buildings (which are much better as of Gods and Kings). Granted, for an offensive war, encouraging the production of defensive buildings may be questionable, but something to keep in mind is that they ARE now maintenance-free, so they may be worth building for the happiness alone~

Piety
Piety used to be only used pretty much for cultural victories, and that's probably mostly what it will still be used for. Before it emphasized happiness and culture, but now it plays well into the faith system for religion. The two times to pick Piety is if you are planning to make a wide empire that utilizes religion for great effect and need to generate lots of faith, or a tall empire that is heading for a cultural victory. Unfortunately I feel this tree is hard to use well and should probably be avoided unless you have a very specific strategy for utilizing it or are going for a cultural victory.

Piety Policy Tree (Unlocks in the Classical Era) Exclusive With Rationalism
The opening benefit for adopting Piety allows you to build Shrines and Temples in half the time. This benefits wide empires more than tall, because by the time you have reached the Classical era, chances are you've already built Shrines in your first few cities if you have any intention of going for a religion (unless you decided to skip shrines and go straight for Stonehenge, which isn't terrible).

Once you adopt every policy in the Piety tree, you receive a 20% discount on the purchase of religious units and buildings with Faith (note that this does NOT discount Great People or Holy Warriors), and adds +3 Gold and +3 Culture to the yields of all Holy Sites (the great tile improvement created by Great Prophets). This is best for empires that want to produce and generate lots of faith, as Holy Sites aren't of much use if you don't use Faith much, and it combines nicely with Freedom's finisher, which doubles the Faith yield of the Holy Site (but not the Culture or Gold).

Organized Religion: +1 Faith from Shrines and Temples. More faith from your faith-producing buildings is good if you want to use faith.

Mandate of Heaven: 50% of the the excess happiness in your empire is added to your total culture every turn. First off, this is a global yield so it not affected by local city modifiers. Second, this is honestly of questionable value unless you can manage very high happiness yields constantly throughout the game, which can be difficult. If you can, however, this is a nice benefit for extra culture.

Theocracy: Requires Organized Religion. Temples add +10% gold in the city in which they are built. Nice, but nothing special really. If you're going to build lots of temples anyway you can get some use out of this.

Reformation: Requires Organized Religion. +33% Culture in cities with a World Wonder. Now this is what I'm talking about. Make sure most of your cities have at least one world wonder, and you can get a lot of extra culture with this. Great for cultural victories.

Religious Tolerance: Requires Reformation and Mandate of Heaven. Reduces the Culture cost of future policies by 10%. The other major policy for cultural victories, grab this as early as you can, because you need 30 policies to unlock the Utopia Project.

Patronage
Ahhh, good ole Patronage. Back when Maritime city-states were insanely powerful, this was also insanely powerful. As is, it's still great if you want to ally with a bunch of city-states, and especially useful for the Diplomatic Victory. The tree emphasizes making it easier to build and maintain good relations with city-states, and also adds new benefits to your empire from city-state allies.

Patronage Policy Tree (Unlocks in the Medieval Era)
Adopting the patronage tree will slow the rate of decay of your influence with city-states by 25%. Simple, sweet, effective. Not that this only refers to the natural decay of influence over time, and not influence lost in city-state coups or invasion of their territory.

Adopting every policy in the patronage tree will cause each other civilization in the game to lose influence with city-states 33% faster than usual. And don't think the AI won't take this, because they will! This is again pretty self-explanatory, and makes competition for the favor of city-states favor empires that have invested in patronage.

Philanthropy: Increase the influence granted by gifts of Gold to city-state by 25%. The easiest way to get influence with city-states is to give them some gold, and this means you get more influence with less gold. I don't need to explain this any more, do I?

Aesthetics: Resting point of Influence with city-states is raised by 20. The "resting point" of a city-state is basically your "default" reputation if you have no influence with them. If you've done things to anger the city-state, your influence will slowly increase over time until it reaches this point, and your influence with a city-state will steadily decay over time until it reaches this point at any time it is more than this point. Note that combining Aesthetics with pledging to protect a city-state sets your resting point at 30, which is "Friendly," and gives you some minor bonuses.

Scholasticism: Requires Philanthropy. City-states allies provide you with 25% of the science they create for themselves. Because city-states typically only possess one city and don't micromanage it to full effectiveness, this is not a whole lot of science. But it's nice. More science is never bad.

Cultural Diplomacy: Requires Scholasticism. Quantity of gifted resources from city-states increased by 100%, and happiness from gifted luxuries from city-states increased by 50%. A city-state that is your ally gifts you all of its luxuries and strategic resources. This doubles the amount of strategic resources you receive, and any luxury your city-state provides gives you 6 happiness for your empire instead of the usual 4. Both of these are good things.

Educated Elite: Requires Scholasticism and Aesthetics. City-states will occasionally gift you a great person. Remember when I said free great people were nice? While you can't control what kind of great person you get, they are all generally useful in one way or another, and you get them periodically. This is a very strong policy.

Commerce
Commerce has two focuses, divided into the left branch and the right branch. One focus is in gold generation, and the other is on naval superiority. The policy tree is stronger on water maps, but any civilization can make use of extra gold.

Commerce Policy Tree (Unlocks in the Medieval Era)
Once you adopt the commerce tree, the gold generation of your capital city increases by 25%. This is basically a free bank in your capital city, though of course it will stack with an existing bank. Make sure you put your National Treasury in your capital, as that 8 gold will become 10.

Adopting all policies in the commerce tree increases the gold output of every trading post by 1, and doubles the gold yielded from Trade Missions using Great Merchants. The trading post bonus alone is very good if you use trading posts, as this increases the yield of the improvement to 3 gold with economics! It also makes Great Merchants quite a lucrative investment, so keep that in mind and make sure to use them for trade missions after maxing out this tree.

Naval Tradition: +1 movement for naval units and +1 sight range for military naval units. A free great admiral appears outside the capital city, and the movement of all great admirals is increased by +2. Great admirals are basically the naval equivalent of a great general, and can also be expended to instantly heal all of your nearby naval units. Given that naval units cannot heal outside your territory without a certain promotion, having multiples can be very handy! Oh, and your ships move faster and see farther, which is essential for naval warfare.

Merchant Navy: Requires Naval Tradition. Each coastal city gains +3 production. A very potent boost in water-based maps, as most if not all of your cities may be coastal there, and +3 production is a potent boon. Works best with wide empires, and stacks with Liberty's Republic and Order's Communism for super-productive cities.

Trade Unions: Harbors and Seaports grant +1 gold, and reduces the maintenance cost of roads and railroads by 33%. This grants you extra gold one way or the other, whether your rely on a road network, harbor connections, or both for trade routes.

Mercantilism: Requires Trade Unions. Rush-buying items in cities costs 25% less gold, and your Markets, Minks, Banks, and Stock Exchanges produce +1 Science. This saves you gold when you buy stuff directly, and gives you free science on all your gold-producing buildings to boot. Nice little policy.

Protectionism: Requires Mercantilism. +2 happiness for each luxury resource. This is potentially a lot of extra happiness. Very nice way to finish off the tree. Remember, extra happiness fuels golden ages, and those are very nice indeed.

Rationalism
Rationalism has a very clear focus on science and science-producing buildings. It aims to enhance your output of science and offer other miscellaneous bonuses to your science-producing buildings. Especially great if you're going for a space race, but everyone can use extra science.

Note that this policy tree is exclusive with Piety, in that you cannot receive benefits from both and it is generally unwise to invest in both at the same time. Pick one and stick with it would be my advice.

Rationalism Policy Tree (Unlocks in the Renaissance Era) Exclusive with Piety
Adopting rationalism grants your empire +15% science when happy. Just make sure to keep your happiness up so you benefit from this. Rather silly to run up against the happiness cap when you are dependent on your citizens being happy with this bonus.

Filling out the rationalism tree grants you two free technologies instantly. I should not have to tell you how powerful that is.

Secularism: +2 Science per specalist (I know this is misspelled but I don't want to hit the blacklist feature). Extra science if you run more specalists. Just make sure to run some and you'll benefit from this.

Free Thought: Requires Secularism. +1 Science from Trading Posts and +17% science from Universities. This increases the boost from Universities from 33% to 50%, and adds extra science to every Trading Post. Naturally, this works best with lots of Trading Posts and Universities. It combos especially nicely with jungle cities, as Universities add +2 science to every jungle tile, and you can build a trading post in a jungle without removing the jungle.

Scientific Revolution: Requires Free Thought. +50% Science from Research Agreements. You need to sign research agreements to get use from this, but this makes them quite strong. Combine with the Porcelain Tower wonder for maximum effect.

Humanism: +1 Happiness from Universities, Public Schools, and Observatories. Meh, kind of nice. Combos well with the opener. Keep in mind that Observatories can only be built in cities adjacent to a mountain, so this might not end up being very much happiness unless you are a wide empire with science buildings everywhere.

Sovereignty: Requires Humanism. +1 Gold from all science buildings. This can add a lot of gold, but mostly it just offsets the maintenance cost of the buildings. Quite nice for China and their Paper Makers. Should work for scientific wonders like the all-important National College too.

Note on the Industrial Era Policy Trees
These three trees: Order, Autocracy, and Freedom, are mutually exclusive. You can only have one of these three trees in effect at a time, much like the Piety/Rationalism split. The focus split is similar to the three ancient era trees. Choose wisely.

Order
Order provides benefits generally on a "per city" basis, making it well suited for wide empires with lots of cities. If you opened with Liberty, I see Order as a natural extension. Liberty empowers you to settle lots of cities, and Order lets you get the most out of those cities.

Order Policy Tree (Unlocks in the Industrial Era) Exclusive with Autocracy and Freedom
Adopting order adds +1 happiness per city. Great, as settling lots of cities can cause happiness problems, and this helps to alleviate somewhat.

When you obtain all policies in the order tree... well. In my opinion, this is the most powerful benefit in the game: all of your cities gain +2 food, +2 production, +2 gold, +2 science, and +2 culture. Yeah... that's a massive increase in yields if you have a wide empire. Congratulations, all of your cities are now mega-cities.

United Front: Militaristic city-states grant you units twice as often when you are at war with a common foe. This is a nice benefit, but nothing too amazing. Since your city-state allies automatically declare war on anyone you declare war on, this is generally easy to make use of.

Nationalism: +15% combat bonus when fighting in friendly territory. Provides a powerful combat bonus for defensive wars, or going to war to defend your allies. Nice, but combat is not the primary focus of this tree.

Planned Economy: Build factories in half the time, +25% local science yield for cities with factories. This can combine with a factory push in your cities for big yields! Factories are expensive but greatly enhance the productive capacity of a city, and you get a free science boost on top of that. Note a city needs a workshop before it can produce a factory.

Socialism: Requires Planned Economy. Gold maintenance of buildings reduced by 15%. This is not a big discount, but it's nice when you have lots of cities with a few buildings, none of which are making huge amounts of gold. But what you really want is...

Communism: Requires Socialism. +2 Production for every city and +1 Production to every Mine and Quarry. Again, this boost scales with the number of cities, and adds a lot of production to your empire--both the city bonus and the boost to mines and quarries are useful. Very useful.

Autocracy
This is a warmonger's trait and is even more powerful than honor. Combined with honor, and your army will be exceptionally dangerous and you will have massive gains with conquest. The ultimate tree for warmongers.

Autocracy Policy Tree (Unlocks in the Industrial Era) Exclusive with Order and Freedom
Adopting autocracy reduces unit maintenance by 33%, and you gain 10 points of culture as plunder for every point of culture a captured city produces per turn when you capture it. Do NOT underestimate how much culture the latter benefit provides, I was getting hundreds of culture points per city captured. And you pay less to maintain your massive army to boot. Very potent.

What's more, maxing out the autocracy tree provides a +25% combat strength bonus for all of your military units for X turns (based on game speed, 50 turns at epic speed, so a very long time). Um, yeah. By the time you max this tree out with honor, your military units can be so much more powerful than an enemy's that conquest is a cakewalk even if they have better units.

Populism: Wounded units deal 25% more damage than usual. Units that are missing health are generally weaker in combat than the same unit at full strength, this policy evens the gap. Very powerful in the hands of Japan, as Oda Nobunaga's Bushido trait makes wounded units fight at full strength, so your wounded units are actually even more deadly than when they are at full health.

Militarism: Reduces the cost of buying units with gold by 33%. Buy more units for less gold. Simple, effective, stacks with the Commerce policy.

Police State: Requires Militarism. +3 Local City Happiness from Courthouses. Build courthouses in half the normal time. Enemy spies are 25% less effective, and your spies are 25% more effective in killing enemy spies in counter-intelligence operations. This policy makes your conquests potentially provide even more happiness than your own cities, due to how courthouses work. It also makes you almost impossible to steal technologies from. Both of these things are good.

Fascism: Requires Militarism and Populism. Quantity of strategic resources produced by your empire increased by 100%, and great generals get +2 movement. Ehhh... extra movement for great generals is nice, but seems underpowered this late in the game. The quantity thing can be useful if you have limited amounts of aluminum and oil (even coal perhaps), and the way this is worded, SHOULD provide extra aluminum from recycling centers. If you're short on such resources to fuel your war machine, this could come in handy.

Total War: Requires Fascism and Police State. +25% production when producing military units and all military units being with +15 experience points. The last policy available in this tree is a huge increase in the speed at which you produce units, and makes your new units even more potent. If you take this policy, you better go conquer the world afterwards.

Freedom
Freedom is a bit of an oddball tree, but its primary focus is on great people. Obstinately, it works best for "tall" empires or empires seeking a cultural victory. Make sure you run lots of specalists if you go Freedom.

Freedom Policy Tree (Unlocks in the Industrial Era) Exclusive with Order and Autocracy
Adopting freedom increases the great person generation of your cities by 25%. Nice, simple, effective, but you really need to run specalists to get the full use of this. So do that.

When you adopt all policies in freedom, the length of your golden ages is extended by 50%, and the base tile yields of all Great Tile Improvements (that is, the improvements uniquely created by Great People) are doubled. Note that additional bonuses from technologies or other policies do not double. Make sure you set up a lot of great tile improvements before you pop this--Landmarks, Academies, Manufacturies, Custom Houses, and Holy Sites.

Constitution: +2 Culture per World Wonder. A bit weird... but a nice followup for someone who has gone Tradition -> Piety -> Constitution. Tradition for +15% production of Wonders, Piety for +33% Culture in each city with a World Wonder, then pick this up for a bit more culture. But really, the bonus is rather small.

Universal Suffrage: Combat strength of cities increased by 33%. Your cities will be even harder to crack now, especially with Walls or other defensive structures. Nice, but one wonders how often you really need this.

Civil Society: Specalists consume half the normal amount of good. Now we're talking. One of the biggest drawbacks to specalists is that they cannot produce food like citizens working the land, but still consume food--now they consume a lot less.

Free Speech: Requires Constitution. 8 units are maintenance-free. This policy is weirdly named in my opinion, as I don't have any idea what this has to do with "free speech," but it can save you a fair amount of gold in a small empire with a smaller fighting force. Eh. Could do worse, I suppose...

Democracy: Requires Civil Society. Specalists provide only half the amount of unhappiness as normal population. Okay, this is a very nice bonus to the specalist-driven, great-person producing empire. This will give you a lot of happiness if you are using Freedom correctly. And that happiness means more golden ages and therefore more benefits. Yes, you want this.

One Last Note on Faith and Great People
As of Gods and Kings, you can use faith points to purchase great people after adopting certain policy trees. The cost of the great person increases with every great person purchased, but it does not slow down your normal rate of great person generation. This is a very potent use for faith in the late game. Note that you can always purchase great prophets with faith, you just have to do so manually after the advent of the Industrial Era.

Note you only need to adopt the tree itself to purchase the great people, you do not need to fill it out. Furthermore, the great person cost increases only for the same kind of great person--purchasing a great scientist with faith does not increase the cost of purchasing great engineers.

Order: Allows the purchase of Great Engineers with Faith.
Autocracy: Allows the purchase of Great Generals and Great Admirals with Faith.
Freedom: Allows the purchase of Great Artists with Faith.
Rationalism: Allows the purchase of Great Scientists with Faith.
Commerce: Allows the purchase of Great Merchants with Faith.

The top five policy trees do not unlock the purchase of any great people with faith.

In this post, I'll give a basic overview of the combat system in Civilization 5, since it is vastly different than its predecessors in many ways. There are four major differences:

1) One unit per tile.
2) Hex-based grid rather than a square grid.
3) Combat is often not lethal for either side in an individual combat occurrence. Meaning if you attack an archer with a warrior, you're unlikely to completely destroy the archer in a single turn, unless the archer is already damaged.
4) Certain units do "ranged" combat, which means they do not move to attack and take no damage from the attacker, most of the time from multiple hexes away.

Unit Categories

Melee Units: Melee units are the most straight-forward unit, beginning with your starting warrior and scout. Melee units attack directly and generally possess a high combat strength, with those that go on the swordsman -> longswordman line typically being more powerful than the spearman -> pikeman line. However, it should be noted that the pike line does not require iron to build (unlike swordsman and longswordsman), and get combat bonuses against mounted units. It is important to note that there is a bit of a split as pikemen upgrade to lancers, which upgrade to lancers (a mounted unit), and eventually upgrade to anti-tank guns and helicopter units, while swordsman follow the path straight into mechanized infantry. One last important thing is that ranged units and helicopter gunships cannot take cities. Even if you wear a city down to 0 health, you need a melee unit (or a naval melee unit) to capture the city.

Archery Units: Archery units are ranged units, meaning that they can deal damage to units without suffering damage in the process. Archers, composite bowmen, and crossbowmen also possess a 2-tile range, meaning they can attack distant foes (though keep in mind that they cannot fire over hills or forests unless they are on a hill themselves). Eventually, crossbowmen upgrade to gatling guns and machine guns, which still attack using a "ranged attack," but only have a 1-tile range. Meaning they attack adjacent enemies but take no damage in the process. This is made up for by the raw power of gatling guns and machine guns, which is considerable for the era. Furthermore, they can gain access to +range promotions which will allow them to attack from 2 tiles away later on, or even promotions that allow them to attack twice in a turn.

Siege Units: Siege units, like archery units, are ranged units. However, they use movement to "set up" before they can fire. Siege units are especially effective against cities, and can take a promotion to be even more effective against cities. With cities now being tougher than before, most attacks against opponents with technological parity will require some siege to take down. Rocket artillery later in the game do not need to set up to fire, but it should also be noted that siege units are often eclipsed by the increasing power of air units and naval units in the very late game.

Mounted Units: Mounted units are very fast, but take a penalty when attacking cities. However, they can capture cities, and function largely as melee units with high speed. Mounted units receive no defensive bonuses from terrain, and eventually upgrade along the tank line. They are unlikely to form the front line of your military because of their weakness on the defense, but may be used for support, sneaking around to hit the enemy's back-line, or scouting. Functionally, tanks and modern armor work the same as mounted units, however their raw power makes them much more suited to the front lines with air support. It should also be noted that certain mounted units are ranged, mostly certain unique units and the chariot archer, and naturally these units cannot take cities.

Naval Units: There are two kinds of naval units: melee naval units, which function like land-based melee units, and ranged naval units, which function like land-based ranged units. The primary difference is that all ships typically have a high movement speed, and the seas typically have less bonuses for terrain than land. Melee naval units can capture cities and typically have overwhelming power in ship-to-ship combat, but ranged naval units are more suited for bombarding land-based units and weakening cities/other naval units from a distance. Basically, melee naval units should work to protect ranged naval units from attack. It should be noted there is a rather steep drop-off with the introduction of ironclads, which absolutely wreck a wooden navy. Since any unit can embark, but are usually weaker when embarked, you will often also use your navy to escort a land-based invasion force for its journey overseas. Proper naval units will almost always decimate embarked units, but note that embarked units can stack with combat naval units for protection. Finally, submarine units are ranged naval units that are unique because they remain invisible unless they have attacked that round or are adjacent to another naval unit. Submarines can also pass under glacial terrain, and the more advance submarines (nuclear submarines) can carry missile units. Submarines cannot attack land units, unlike other ranged naval units (though obviously missile units carried on nuclear submarines can attack land units). Carriers do not attack at all, but simply serve as mobile bases for aircraft and missile units.

Air Units: There are two kinds of air units, fighters and bombers (helicopters function the same as melee land units, except they cannot take cities). Both types of air units can make "ground attacks," though bombers are significantly stronger and can take promotions to make them stronger against cities. Fighters, in addition, can take on "intercept" and "air sweep" missions. An intercept mission protects the fighter's operational range against other air units, meaning that whenever a bomber or another fighter attempts to attack a ground unit or city that is within the intercept radius, the fighter will engage the offending unit instead. An air sweep mission will attempt to intentionally draw fire and engage potential interceptors or water/ground-based anti-air fire (from destroyers or anti-aircraft guns, which can 'intercept' aircraft that target nearby tiles). This effectively "clears the way" for your bombers, as each unit can only intercept once per turn. A fighter performing an air-sweep will dogfight a enemy fighter performing an intercept mission in, which can cause major damage to both depending on combat strength and promotions, but most importantly allows the bombers to follow up with ground attacks safely (provided there isn't more air cover). Aircraft do not occupy tiles on the grid as per normal, they are "based" in cities or carriers and perform missions from there. Note that stealth bombers have a very good chance of avoiding any form of interception, but cannot be based on aircraft carriers (not to mention come very late in the tech tree).

Missile Units: Missile units function mostly like air units, in that they are based in cities or carriers. However, missile units are one-shot units that are lost when they complete their mission. Cruise missiles are the simplest form, they are very cheap and can be based on carriers, missile cruisers, or nuclear submarines. Atomic bombs are also one-shot missile units, but they pack heavy destructive power, and can only be based in carriers or cities. Nuclear missiles are essentially more advanced atomic bombs that can be carried by missile cruisers or nuclear submarines in addition to cities and carriers. Nuclear weapons are extraordinarily powerful and deal damage in an area of effect, being the only kind of unit in the game that can do so, and also simultaneously pillages any tile improvements in the area of effect, and leave behind fallout which renders the affected tiles basically worthless until they are cleaned up by a worker. Cities hit by a nuclear attack lose population, though an atomic bomb cannot completely destroy a city. However, nuclear missiles can--hit a weak enough city with a nuclear missile, and it might be wiped off the map completely. It should go without saying that units and cities within the area of effect of a nuclear explosion also take massive damage.

Basic Strategies to Military Success

When you go to war, your basic offensive objective should be to surround the enemy city, and bombard it with ranged units and siege, then finish it off with one or several attacks by melee units. If you're on the defensive, you should try to make it as difficult as possible for your enemy to do this. In other words, do not just hide behind your city walls and attack units from safety, try to surround your own city with units to make the enemy first dislodge your defenders before advancing. The more time you can buy for your city, the more times you can bombard the enemies with your own ranged units and your cities. If your enemy has very little in the way of melee units and is ranged-heavy, mounted units will make a very effective counter to their strategy, simply charge their line head-on.

This is why it's important to have a good unit mix, it may be tempting to simply go full ranged units, but you can exploit a strategy tipped one way or the other easily. An attack force with only melee units will have a hard time actually bringing all of its forces to bear because of one-unit-per-tile. Focus-fire tactics are extremely effective--if you attack a unit, try to attack it in numbers and finish it off, a weakened unit can retreat and heal up to plague you later on, or earn enough promotions to be stronger (or use an instant-heal promotion). Likewise, if the enemy attacks and leaves your units wounded, have them retreat to heal up--they did their job. It's much easier to survive your city being surrounded by weaker units than full-strength units. Station a ranged unit in your city and use the ranged unit + city bombardment on the same unit to eliminate weakened units, so they can't attack at all. If they have very few units of a particular type, like siege units or melee units, you may wish to focus fire those first--after all, your city can't be captured by a bunch of ranged units, even if they bring it down to 0 health!

Asymmetric warfare is now much more important, because of the fact that melee naval units can now capture cities. On the right kind of map, even if you're weak on land, you may be able to win a war by going to sea! You can even bombard enemy units from the coast with naval ranged units if you rule the seas, tipping the scales severely in your favor. It should go without saying that technological superiority is paramount, but there are lots of ways to hold out against a more powerful foe if you're on the defensive. It should also be noted that the AI is somewhat tactically inept, and will not react well to having its strategies foiled (it tends to go into full retreat if you bloody their attack force). Having air power when your opponent does not can turn a war completely around as well. One-unit-per-tile also makes choke points more common and vitally important, and you can defend these points to protect your territory, even using a great general to create a defensive citadel at a choke point to double the defensive boost of a unit stationed on the tile can create a boundary that no enemy unit can pass. Likewise, if the enemy is foiling you with a choke point, look for a way around it, by sea perhaps, or use air power to dislodge units protecting the choke.

Walls and other defensive structures (it progresses steadily along a line: Walls to Castles to Arsenals to Military Bases) grant a city bonus hit points and bonus combat strength. Cities with defensive structures are even more difficult to take than normal, and more resistant to siege attacks. If you suspect the enemy will hit a particular city, try to build defensive structures here quickly, cash-rushing if necessary. Sell off your resources to AI for the gold if you need to. If you have a few cities that form a "front line" for your civilization, it might be prudent to build defensive structures here in advance, and use sentry units such as scouts or mounted units to keep an eye on enemy movements--it should go without saying that it's easier to defend against something you know that's coming than a complete surprise attack. When moving your army, try to maintain a "line" of melee units ahead of your ranged units and siege, since ranged units are typically weak against direct attack. The same principles apply in naval combat now, and you may wish to use your destroyers and submarines to scout ahead for enemy submarines.

It's hard to prepare for nuclear warfare, but don't group your units up in tight clusters if you suspect the enemy might have nuclear weapons. They will hit your grouped up units, they will hit your city that had 6 aircraft stationed in it and destroy them all, they will obliterate your tightly-packed fleet escorting an embarked invasion force. Basically, if your opponent has nukes, be very careful about attacking them.

If anyone had any questions about anything CiV-related, feel free to ask me~ Maybe next time I'll cover Religion~

This is a short post, but I'm going to show you a neat little micro trick here~ Only those who are interested in micromanaging their cities for maximum possible benefit need apply~

A Short Note on City Growth
When a city reaches a new size, the following happens:

1. Food produced by the city for the current turn is totaled. If it fills the "food box," a new citizen is added to the city.
2. If the city receives a new citizen, it is automatically assigned to a new tile by the "city governor." The city selects a tile appropriate to the city's current "focus," which you can select in the upper right on the individual city's management screen (you need to select this for each city individually, as different cities may have different focuses). The default focus prioritizes city growth unless the empire is unhappy, at which point it prioritizes production but does not allow the city to starve. Puppet cities prioritize gold and their focus cannot be changed.
3. After the new citizen is assigned, all other yields for the turn are added up and benefit accordingly.

Why is this important? What does this mean? One simple fact, my friends~ Any food a newly assigned citizen produces is essentially "wasted," or not added to the city's total for the current turn, but any other yield the citizen would produce is not wasted.

Because you cannot assign the new citizen manually before the yield is added to your total, you are at the mercy of the city governor to pick a new tile for the citizen to work. Naturally, it would be much better if the citizen works a high production/gold tile rather than a high food tile, but chances are if you leave the city on "default focus" then the citizen will be added to a food tile if one is available. Of course, you can't simply switch all of your cities to a production focus, because then they won't grow!

What's the best way to use this to get maximum benefit from your citizens?
1. Have a city set to production focus, but micromanage the tiles. Lock high food tiles so they are used, as the governor will not override any locked tiles.
2. If appropriate to the city, you can do the same as the above, but set it to gold focus, science focus, etc. Production focus is the most generally useful, but any other than food will work~
3. Whenever a city grows (you will receive a notification as to when a city grows to a new size, which is handy for this), check the city and move the citizen as appropriate~ Keep in mind that this means you WILL need to check and recheck on your cities to make sure the citizens are working tiles as appropriate to your current situation in the game~

The benefits from following this strategy can add up over the course of the game, and I believe it will help you learn the deeper mechanics of CiV better~ With that said, it's hardly essential, just a lil' trick I picked up~

I'm going to repost this strategy because it's a really good one that helped me win an Emperor-level game fairly easily, remaining the dominant world power throughout~ Please note that I did not create this strategy, I'm just reformatting it and retyping it from the above thread~

The basic idea is to quickly rush out to four cities while taking tradition policies. Once you have four cities, you can expand further, or build up a military and conquer a neighbor for puppet states. This strategy is actually quite flexible, as I slipped in the Great Library instead of a Library at the capital, using it to grab Iron Working, because I had marble at my capital~

Anyways, for the start of the game, here are your build/tech/policy orders:

Capital City: Scout > Worker > Shrine > Warrior > Settler (try to reach 3 or 4 population before building your settler) > Archer > Archer/Library
2nd City: Archer/Shrine > Archer
3rd City: Archer > Shrine
4th City: Archer > Shrine
Notes on Build Orders: You may settle your second city before Archery, you should either build a Warrior or a Shrine in this case. Try to settle your second city on top of a luxury if possible. When your Worker pops, improve nearby luxuries as fast as you can and immediately sell them to the AI (an AI at good relations with you should buy your luxury for 240 gold at normal speed, 360 on epic). Send your scout and prioritize meeting other Civs and City-states over fighting barbs. You want as many trading partners as possible, and city-states give you gold and faith when you meet them. Plus, national wonders are effectively free happiness if you can find them. When scouting, use your starting warrior to scout out nearby potential city sites, but don't wander off too far because it's a very good idea to use your warrior to cover your worker from opportunistic barbarians. If you can steal a worker from a city-state it's usually a good idea to do so at higher levels, even better if the barb takes it first and you can take it from the barbs (don't give it back, you won't suffer a penalty).

[/b]Buy Order:[/b] Settler > Settler > Worker > Upgrades (+more workers if needed)
Notes on Buying: This is why you are selling your luxuries. Hopefully your trade deal will end and you'll get the luxury back about the time you start brushing up against your happy cap. Something to note is that at lower difficulties you probably will not see AIs with a lot of money to give you, but you also won't need as many archers. You can build another settler after your first archer in the capital that case, and change your second city build to a worker after the archer.

Policy Order: Tradition > Legalism > Monarchy > Landed Elite > Aristocracy > Oligarchy
Notes on Policies: Monarchy before Landed Elite because you want to start accumulating gold asap. Aristocracy can come before Landed Elite if you plan to go for the Great Library (often worth it at Emperor or below), but you only need to have it in time for National College. Oligarchy comes last to get you the very useful full tradition tree benefit of free aqueducts and +15% growth.

Tech Order: Animal Husbandry OR Mining > Pottery > Luxury Tech > Luxury Tech > Archery > Luxury Tech > Beeline Construction OR Writing/Philosophy
Notes on Tech Order: Your tech order varies based on the terrain near your capital. If you need to chop forests to get to your luxuries, pick mining first, but if your luxuries are trapping-based (ivory), then go animal husbandry first. If you're lucky enough to have mining-based luxuries (silver, gold, copper, gems), then you have more flexibility. The idea is to make sure your worker has what it needs to improve a luxury as soon as it finishes building. To this end, "luxury techs" can also include masonry or bronze working if you need to clear jungles or marshes to get access to your nearby luxuries. Obviously Mayans can hold off on Archery if you prefer, since you don't need it to build their unique archers. Construction is the better choice at immortal+ because you want composite bowmen for defense asap (upgrade your archers), but at emperor or lower archers should be sufficient to hold off potential invaders.

The Next Phase
After you've got your first cities out, keep a sharp eye out for aggressive neighbors and start using your archers to pick off nearby barbarians, particularly encampments targeted by city-states.

Tech Path: Construction > Writing > Philosophy > Bronze Working > Iron Working > Metal Casting
Notes: You begin where ever you ended up before. Once you have construction, upgrade your archers to composite bowmen. Continue to sell off extra luxuries as your happiness allows (and continue to improve luxuries with your workers). Don't do luxury for luxury trades unless you are very close to unhappiness.

Notes on Build Path: Once you have writing, it's important to get a library in every city and begin constructing the National College in your capital city. If you're confident enough, you can go for the Great Library in your capital (I recommend Iron Working as the free tech, it's the most expensive classical era tech). National College should go in your capital or what you feel your strongest science city will be (if you managed to settle a city in a jungle next to a mountain, it may have more long-term science potential than your capital). Try to wait until you have Aristocracy, then start on the National College. In the meantime, granaries should go in extra cities, with extra workers and military units bought or constructed as needed (you probably will not need more than 3-5 workers for now). After you discover iron working, look for nearby iron. It may be worth delaying founding your fourth city until you know where nearby iron is, particularly if you're going to grab it with the great library. But that's optional. If you do have iron, build some warriors while you hook it up so you can later upgrade them to swordsmen (this is more efficient than building or buying swordsmen from scratch). If you plan to expand aggressively soon, these will come in handy. After granaries, workshops in every city. Your cities will be very powerful with this base to work with.

Thereafter: Once your workshops are in place and Metal Casting is discovered, there are lots of things you can do. One good plan is to beeline Education asap, then go for Acoustics or Astronomy to break into the Renaissance Era. If you can reach the Renaissance Era at the same time you choose your new policy (after Oligarchy), you can immediately start putting points into the left side of Rationalism for a very powerful research boost (head straight down the left side to Scientific Revolution). If you can't make it to Rationalism in time, consider the 1 point going into Commerce or Patronage. If you can land the Oracle at the same time you break into the Renaissance, well, congrats, you're going to break the game~ Once your Universities are in place, start running Scientist specialists in every city to get Great Scientists, especially if you're a civilization like Korea or Babylon! Alternatively, you can head down the military line of Civil Service/Machinery/Physics/Steel and conquer a neighbor, if you think your swordsmen and composite bowmen aren't up to the task. A handful (3-5) of puppet cities will supplement your tradition opening well. And while this isn't the most favorable strategy to further rapid expansion past four cities, that may be the best option if there's a lot of unsettled land nearby, even though your new cities won't get most of the benefits of tradition. Note that this will slow down your social policy acquisition significantly unless you follow this up by going into Liberty, but then that will cost you at least 3 social policies which will delay any further social policies anyway. Probably not worth it unless you're dedicated to expanding a lot further and fully fill out liberty. As always, the left side of Commerce is a very nice tree for water-based maps, particularly if your cities are coastal.

The main thing to wonder is if you have AI cities bordering you at this point! If you do, it's very likely they will attack you sooner or later, and it's better to take them out as soon as you have an edge~ If you have swordsmen and composite bowmen, and the AI doesn't have pikemen yet, then you have a good shot at taking them out~ If you don't have iron, you'll have to do with pikemen. Try to fight the AI's army out of range of their cities if at all possible, eliminating as many units as possible before making your push. If the AI is more technologically advanced than you don't bother, just try to hold the line with pikemen and composite bowmen/crossbowmen. Note that once you go on the offensive, you may get the "warmonger" label from Civs that are aware of it, which can make finding trading partners difficult. Try to avoid completely eliminating a Civilization if at all possible, leave them with one or more crappy cities and sign peace instead, as there is a significant diplomatic penalty for eliminating a Civ from the game~ You can also let an ally or city-state finish off a wounded opponent. Note that on a continents map, Civs that are unaware of your war will probably not hold it against you. However, if you stole a worker from a city-state, any Civs nearby may still hold that against you so be careful.

Religion

Religion in Civilization 5: Religion in CiV works a lot differently than religion in CIV. The benefits you get are much more customizable. However, there are a limited number of religions that can be founded, and each belief can only belong to a singular religion. Therefore, the "first come first serve" rule of Civ4's religion founding applies on a much wider scale in Civ5. The first civs to found their pantheons, start a religion, or enhance their religion are most likely to get the benefits that help their civilization the most. Therefore, if you want to get the most out of your religion, you should invest early and make a good investment. I'm not going to lie, some of it also comes down to luck. Getting free faith from ruins or meeting a couple of religious city-states first can expedite the founding of your pantheon tremendously.

Faith: Faith is the new yield in Gods and Kings that powers religion. Faith is found in ancient ruins, given to you by religious city-states, and produced by buildings. Tiles can also give you some faith if you have certain religious beliefs. For the purposes of this post, "religious beliefs" can refer to any belief that involves religion, including those from a pantheon. Anyways, you need a certain amount of faith to found a pantheon, which is the first step towards founding your own religion. Note that the cost of creating your pantheon goes up with every currently existing pantheon in the world. So if someone else starts worshiping a pantheon before you establish yours, the cost of yours goes up. When you start worshiping a pantheon, you lose all of the faith you have built up and need to start over from scratch.

After you have established your pantheon, further faith is counted towards a great prophet, the new great person introduced in Gods and Kings. When you generate a great prophet via any means, including the Hagia Sopia or the Mayan Long Count, the faith cost of future great prophets increases. You can mouseover the faith counter to see how much faith is needed before you gain a chance to get a great prophet. Note that before the Industrial Era, you are not guaranteed to get a great prophet even if you have enough faith, it's somewhat random. You'll have a good chance each turn, and eventually you'll pop one. When you do pop one, you again lose all faith you have built up.

From the Industrial Era onwards, you can purchase great prophets directly with faith. You can also purchase other great people with faith, as described in the social policy section. Note that though each purchase of a great person with faith increases the faith cost for purchasing further great people of the same type, it does not increase the normal counter for generating great people naturally with great person points from wonders and specialests.

Great Prophet: A great prophet is necessary to found a religion. One subtlety people often miss is that you can only use a great prophet to found a religion if it is in a city, and the city you use becomes the holy city for that religion. Most people and AI put it in their capital city, but you don't have to. A holy city will always return to following the religion founded in it, even if it is temporarily converted to another religion (though it may take many turns). A great prophet can also act as a super-missionary with 4 spreads, instantly converting an adjacent city to its religion and removing followers of other religions from the city. Note that even though it has 4 spreads, if you use the great prophet to spread the religion even once, then it cannot be used for founding/enhancing a religion or making a religious shrine. Like all great people, once the great prophet is used (or runs out of spreads of religion), it disappears. Note that whenever a great prophet is generated by a civilization that already has a religion present, it is attached to that religion and can only spread or enhance that religion. This means if you capture an enemy great prophet, it will spread the enemy's religion if you use it rather than your own. After the maximum number of religions have been founded, you cannot use a great prophet to found a religion. You can use a great prophet to convert rival AI cities to your religion, though this incurs a diplomatic penalty with that AI leader (and they will immediately ask you to cease, quickly despising you if you do not) if they have founded their own religion.

A great prophet is necessary to found a religion, and when your religion is already founded, another great prophet can be used to "enhance" the religion, which adds two additional beliefs. You can only enhance a religion if your great prophet is present in the religion's holy city--this means you cannot enhance a religion founded by another civilization unless you capture their holy city (since your units cannot enter the cities of other civilizations). Another use for a great prophet is to construct a "holy site" on a tile, much like the great improvements of other great people. The holy site generates a large amount of faith when worked by a citizen.

Beliefs: There are a few different kinds of beliefs, which you may select when you reach certain milestones in developing your religion.

Pantheon Belief: Selected when you found a pantheon. Many cause particular types of terrain to generate faith. Others add faith or culture to tile improvements, while others have more general functions.

Founder Belief: Only the founder of a religion gains this benefit. Often keyed to providing the founder with gold or other benefits on converting cities to this religion, or gaining followers.

Enhancer Belief: Only the civilization that enhances the religion gains this benefit. Most of them are aimed at aiding you in spreading the religion.

Follower Beliefs: You select two of these, once when you found a religion, and once when you enhance it. Anyone who follows this religion gets these benefits, including your rivals! Note that the religion must be the "dominant religion" in the city to get any benefit, even if it's based on follower. In other words, you receive no bonus from Religions Community (1% production per follower), even if there are several followers of the religion in the city, unless the religion that Religious Community is tied to is dominant in the city.

Bonus Belief: Only the Byzantine civilization gains this, when founding a religion. The bonus belief is selected from any of the still-open beliefs, including enhancer beliefs, which are not normally available when founding a religion! This can lead to you gaining two pantheon, founder, or enhancer beliefs tied to a religion, or three follower beliefs, which is not possible for any other civilization.

Going into individual religious beliefs and how they benefit what kind of civilization would be tedious. Let me sum it up: terrain-based pantheon benefits are very good for generating extra faith towards a religion, if you intend to work those tiles (don't pick the tundra-faith boost unless you're actually going to be working tundra tiles, like hills). Fertility rights, religious community, monument to the gods, swords to plowshares, and tithe are the best benefits for a "tall" empire, while beliefs like messenger of the gods, terrain/resource-based benefits, church property, ceremonial burial, initiation rites, and guruship are best for "wide" empires. In particular, wide empires that invest in piety will see the most use out of most religious buildings, while tall empires that invest in piety are most likely going for a cultural victory (there's no real reason to go tradition/piety unless you're going for a cultural victory, piety by and large rewards wide empires) and will appreciate cathedrals, which give an artist specialist slot. You can check this thread for more details on different kinds of empires and what religious beliefs are most beneficial to them. http://forums.civfanatics.com/showthread.php?t=469176