The basic gameplay concept of Civilization IV is the same as that in all other Civ-like games: you take charge as the leader of a band of wandering people and set out to found an an empire of cities, armies and scientists with as goal to eventually rule the world. Compared to Civ3, Civilization IV will introduce some new concepts, such as Religion and Great People, change some existing ones, such as combat and technology, and leave others pretty much the same, such as diplomacy and Wonders. Civlization IV: Warlords adds in addition to that the concept of Vassal States.

Civilization IV is part of the Civilization franchise, and a franchise can usually only be successful if it makes conservatively changes between iterations. So Civ4 is not extremely different from Civ3 -- it will be a conservative sequel. According to Soren Johnson, a good rule of thumb for a franchise is "1/3 old, 1/3 improved, 1/3 new". He believes that the best way to improve on Civ3 is to choose a focus and really improve the game in only a single (or a few) area(s), rather than trying to change a little bit of everything. That doesn't mean that Civ4 will play the same as Civ3, while the essence of previous games has remained, many details have changed and the step from Civ3 to Civ4 is the biggest and most ambitious one the series has made to date....

Barry Caudill, Civilization IV Senior Producer on Promotions In Civ III, unit promotions involved rising in level from green to veteran to elite and each level added a hit point and a small combat bonus. For Civ IV, we decided to build on this system and make promotions much more intricate and customizable by allowing players to choose from over 20 different bonus types (many with multiple levels, for a total of over 40 available promotions). The numbers and types of available bonuses differ per unit type and only land and sea military units can acquire them (air units are different). Here’s how it works: Units can gain experience points in several ways. Successful combat is the primary means (note: you get more points from attacking than you do from defending) and the units must be victorious in a unit-to-unit combat, therefore there are no XPs (experience points) from bombarding fortifications or if the opposing unit withdraws. Units can also gain experience from a Barracks (land units), a Drydock (sea units) or a goody hut. In addition, some wonders, civic choices, and leader traits can give units extra experience or even certain promotions. As you might expect, the first promotion comes rather quickly, while each subsequent promotion requires more XP. The promotions take on many different forms. Some are simple like Combat (5 levels), which gives progressive strength bonuses with some extra healing at the top levels, or City Raider/City Garrison which gives the unit bonuses only when attacking/defending a city. Others give bonuses against a certain type of unit. For instance, Cover gives a bonus against Archery units and Formation gives a similar bonus against Mounted units, while Shock does the same against melee units. Some promotions give bonuses based on the type of terrain in which the unit resides. If your opponent has a lot of woods or jungle, you might choose Woodsman, or if your opponent is surrounded by hills you might choose Guerilla. All in all, the new promotion system in Civ IV offers players a plethora of interesting new decisions, excellent customization options, and an extra layer of strategy. That’s how we spell fun at Firaxis Games. The final aspect of gameplay is for many players also the most important one: combat. If diplomacy fails, or if you're just feeling like kicking some arse, you can declare war on an opponent and take what you want from him/her by force. To do so, you need to build an army of Units and use those to attack enemy Units and Cities, or to pillage his/her Improvements and as such damage his/her economy. But even if you don't want to go to war, you'll still need to build an army: if someone else declares war to you, you'll need to be able to defend yourself. And even if other nations don't declare war, Barbarians might invade you. Posting sentry guards along your borders and fortifying Units inside your Cities to guard them is a wise precaution. When expanding your empire peacefully, you will need to explore the lands around you to find suitable locations to build new Cities, and you'll need to send out Settlers to actually found those Cities (along with escorts to protect them). You need to build Units for these purposes as well. And of course, in order to expand to other continents (either peacefully or aggressively), you will need a navy. Units in Civilization IV function by and large in the same way as in Civ3, but there are some notable differences as well. Most Units still have the basic commands: move, attack, fortify, pillage, disband, sentry, (un)board transport, skip turn. Civilization IV offers one additional standard command: explore, which lets the Unit automatically explore the map. Another minor change is that Units now get a bigger defensive bonus the longer they are fortified. After two turns fortified, the bonus is 10%, after five turns it's 25%. That is also the maximum fortification bonus. A bigger difference is in Unit stats: in Civ3 Units had both an attack and a defense value, in Civ4 they only have one generic Strength value. This Strength value is used both to determine the chance that a Unit hits its opponent in combat (similar to the attack/defense value in previous Civ games) and the amount of damage that is done when a hit is scored (similar to firepower in Civ2). This makes the classic Spearman vs Tank problem much less of an issue, as a Spearman not only has a much smaller chance of doing damage, it also does much less damage. Despite the single Strength value there is still plenty of variation in the combat model, as many Units have much more unique special abilities: Spearmen have a bonus versus Mounted Units, Archers have a bonus defending Cities. Chariots and Cavalry don't get defensive bonuses, but they do have a chance to withdraw from a battle if it's going badly. With this system, every Unit has a specific role in the game and every ...

Diplomacy

The final type of global activity that takes place in Civilization IV is diplomacy: as leader of your empire, you can sit down with the leaders of other empires and exchange technologies, trade resources or sign treaties. Diplomacy works pretty much the same as in Civ3: When you first encounter a civ, its leader is added to the list of known contacts (visible in the bottom-right of the screen). At this point you can always contact him/her for one of three actions: negotiate a new deal, declare war, or view (and renegotiate) existing deals [see image on the right]. When negotiating a new deal, or renegotiating an old one, you're taken to a negotation table [see image below]. There you can negotiate a composite deal that is as complex as you'd like: you can offer Gold for an Advance, a City for a World Map, two Resources and an Advance. You can exchange a Resource for an amount of Gold per turn, sign an Open Borders treaty so your Units can pass through each other's territory, demand a City lest you declare war, etc. You can ask a player to declare war with someone else (even if you're not at war with them yourself), or request them to stop an ongoing war.

While you are compiling a proposal, the animated leaderheads will show you with their facial expressions what they think of your proposal. The general attitude they hold towards you, based on both personality and past behaviour, is also visible (Gracious, Polite, Cautious, Annoyed, Enraged, etc). Of course, you're not the only one who can initiate discussions: rival players can make proposals or demands to you as well, including requests to switch your State Religion or Civics to their personal favourites.

Each AI leader has a distinct personality, this is a major area of focus for development for Firaxis. This personality has a major impact on how they behave in diplomacy: Gandhi will be receptive to peaceful players and generous to relatively weak ones, but he will get angry at aggressive players. Genghis Khan on the other hand will shy away from diplomacy altogether, but when he does engage in it he'll be most receptive to warmongers.

This personality does not only cover their behaviour in terms of what they may demand or how active they are in diplomacy, but also in their facial expressions. Each leader has their own set of expressions: they smile when they are happy, frown when they are upset. Some are very expressive while others are constrained. E.g. Hatshepsut is flirtatious, Louis XIV is vain and arrogant. They will be visibly excited when you make them a good deal and they react upset to less favourable proposals. For others, such as Qin Shi Huang, you can only judge their mood by the elevation of their eyebrows. Julius Caesar litterally gives you a thumbs-up or thumbs-down when responding to a proposal.

Civilization IV: Warlords will introduce a new concept to diplomacy: Vassal States. It's not yet known how this will work.

Espionage

Espionage also exists in Civilization IV and it works pretty much the same as in Civ2: you can build Spy units and send them around the map to infiltrate enemy territory unseen, spy on enemy troop movement, investigate cities, sabotage things, etc -- or use them to expose enemy spies.

Besides sliders and Civics, research is another thing that happens on a global level in Civilization IV. All the Research Points, or Beakers, that every City collects are put together and used to research Advances. These Advances enable new options for the player, such as new Units, Buildings, Civics, Religions, etc and also open the way to new Advances. As such they move game forwards, from Stone Age to Space Age: the technology tree, which determines the relationships between the Advances and what new options each Advance enables, forms the backbone of the entire game. Once the whole tech tree has been researched, a player still has so-called Future Technologies to pursue: each of these gives a bonus to both Health and Happiness....

Barry Caudill, Civilization IV Senior Producer on Civics

Choosing your government has been an interesting part of playing Civilization since the original. In Civ IV, our team at Firaxis has really ramped up this feature and given players many more choices, which bring with them a whole new set of interesting decisions to make and strategies to employ. The new Civics system will allow players to customize their government to fit their current situation and style of play. In the game, when you first open the Civics page you will see 25 options divided into 5 categories. The categories are: government, legal, labor, economy, and religion. Initially, you will be limited to the lowest levels for each (making you a barbaric, decentralized despotism with tribal labor and practicing paganism), but you will unlock more of the choices based on your research. Changing to new Civic forms will have a dramatic effect on the character and success of your civilization. You’ll be able to boost or cut productivity, wealth, and happiness, make choices to increase/decrease the spread of religion, and even affect your ability to produce and maintain a large standing army. Of course, it’s not just as simple as picking all the highest level Civic options. A monarch needs to make the tough decisions. Every choice has an upkeep level assigned to it and you could end up with a really great government that puts you in the poorhouse. In addition, your current circumstances might make it impossible for you to use some of the higher-level choices. The good news for all the leaders out there is that you will be able to play around with all the choices and get an idea of the costs and benefits before you have to “Start the Revolution”. So, the choice is up to you. Will you be a despot or a king? Will your people respond better to a democratically elected ruler or a theocrat? Will you have a free market or is everything state property? Make the best choices and lead your people to their rightful place in history!

Most of what was discussed up until now had to be managed on a City-by-City basis. There are however also settings that apply on a more global level to an entire Civilization, such as Civics and tax sliders. Tax sliders determine how the Gold income of Cities is distributed after fixed costs such as Unit support have been deducted: some of it can go to the treasury, where a player can spend it on things like diplomacy or rush-buying items, some of it can be devoted to research to obtain new Advances, and some of it can used to generate Culture and Happiness. The science tax determines what percentage goes to science, the Culture slider determines what goes to both Culture and Happiness. The remainder goes to the treasury.

Civics are a new feature in Civilization IV and replace the governments feature from earlier games. It works somewhat similarly to SMAC's social engineering. Rather than having fixed government types such as democracy or communism, Civ4 lets you set individual policies on five different terrains: Government, Legal, Labor, Economy, and Religion. Each of these categories will have a total of five different Civic options (for a total of 25) which all have different effects on gameplay and which are unlocked by researching certain Advances. Any combination of Civics from the different categories is possible, effectively creating a total of 3,125 theoretical 'governments'.

Some Civics are very oppressive, such as Slavery (which allows you to rush-build items with population) or Theocracy, while others are very enlightened, such as Emancipation (which causes unhappiness in nations which don't have Emancipation: oppressed people will get jealous of the freedom and civil rights your people enjoy) or Freedom of Religion. So you can be as cruel or kind to your people as you want to be. Every Civic also has an Upkeep Cost associated with it, so you may not always be able to afford every (combination of) Civic(s). Leaders with the Organized trait only pay half of this cost. AI personalities will all have their own preferences regarding Civics options and will try to convince you to implement the same ones, much like with Religions.

Which Civics are available in Civilization IV you can see in the Civilopedia.