Civilization IV follows some of the
4X model of turn-based strategy games, a genre in which players control an
empire and "explore, expand, exploit, and exterminate", by having the player attempt to lead a modest group of
people from a base with initially scarce resources into a successful
empire or
civilization. The condition for winning the game is accomplished through one of the five ways: militarily defeating all other civilizations in the game world, controlling over two-thirds of the game world's land and population, building the first spaceship in the Space Age and sending it to
Alpha Centauri, having the most dominant Culture ratings over other civilizations, or becoming "World Leader" through the
United Nations votes. Additionally, there are multiple game scores for each civilization throughout the game based on the actions of each civilization and a number of factors, allowing for a win condition based on the total of these points if the game timer runs out. The game can be played in multiple modes: as a
single player facing against one or more
computer-controlled opponents, in hot seat mode, or through
online multiplayer games. As with other turn-based strategy video games, the player can customize the look and feel of their game world as well as the difficulty of any game AI players before the game starts. Each map space has a terrain type, such as plains, tundra, or desert, that affects the available resources players can extract from their environments and the movements of certain units through that terrain. The player is then given 18 civilizations to choose from, each with their own
pros and cons, plus a leader avatar, an initial set of civilization technology, and any units unique to that civilization. When the game starts, however, it chooses random locations to place across a predefined square grid map. Like other
strategy games,
Civilization IV has a
fog of war feature, in which unexplored territory remains darkened and territories without any units stationed on its designated square is shaded with darker colors.
Units and combat Most units that the player can generate and use are military units, with certain attributes such as combat strength and movement rate particular to each military type. Each unit can gain
experience through combat, which later translates into promotions that the player can use to assign military units new bonuses. The game features seven religions (Buddhism, Christianity, Confucianism, Hinduism, Islam, Judaism, and Taoism) that are founded by the first civilization to research a certain technology which varies per religion. Religions can then be spread actively through the production of missionary units or passively through means such as trade routes. Religions may be spread to domestic and foreign cities and there is no limit to the number of religions that can be present in any single city. The player may only choose one state religion at a time and all seven offer identical advantages (e.g. cities with the state religion receive bonuses in happiness, and potentially in production, science, gold and even military unit experience points).
Customization Civilization IV is much more open to modification than its predecessors were. Game data and rules are stored in
XML files, and a
Software Development Kit was released in April 2006 to allow customization of
game AI. The map editor supports
Python. The World Builder allows a player to either design a map from scratch or edit a preloaded template map as a starting point for a new game. The player can modify the map by placing and modifying any number of rivers, landmasses, mountains, resources, units, and cities, as well as their attributes such as Culture generation. The World Builder for
Civilization IV is in-game, in contrast to previous
Civilization games where the Map Editor was an external application. More game attributes are stored in XML files, which can be edited with an external text editor or application. On September 20, 2005,
Firaxis Games senior producer Barry Caudill stated that "[e]diting these files will allow players to tweak simple game rules and change or add content. For instance, they can add new unit or building types, change the cost of wonders, or add new civilizations. Players can also change the sounds played at certain times or edit the play list for your soundtrack." ==Production==