Civilization IV uses the 4X empire-building model for turn-based strategy gameplay, in which the player's main objective is to construct a civilization from limited initial resources. Most standard full-length games start the player with a settler unit and/or a city unit in the year 4000 BC. As with other games in the series, there are by default five objectives the player can pursue in order to finish the game: conquering all other civilizations, controlling a supermajority of the game world's land and population, building and sending the first sleeper ship to the Alpha Centauri star system, increasing the "Culture ratings" of at least three different cities to "legendary" levels, or winning a "World Leader" popularity contest by the United Nations. If the time limit for the game is reached and none of the previous goals has been fulfilled by any players including game AI players, the civilization with the highest total game score is declared winner. A large departure from earlier Civilization games is a new graphics engine created from scratch, based on the Gamebryo engine by Numerical Design Limited (NDL).
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 peoples 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 different 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 a total of 18 different 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.
Non-military units include settlers who are used to found cities, workers who are used to improve space and gather resources, spies who can perform counterintelligence and espionage, and religious missionaries who can be sent to convert different civilizations and cities. Also, with the Beyond the Sword expansion, new units are added such as executives, who can spread corporations to new cities. Throughout the game, players may also generate a special unit called a "Great Person". These are named after historical figures and can be used for one-time advantage boosts in various ways; examples include Great Engineers who increase overall production levels and Great Scientists who improve technology. Assigning inhabitants of cities to work as 'specialists' (scientists, engineers, artists, spies, merchants or priests), building certain wonders or discovering certain technologies can improve the rate at which Great People are generated. Great Generals are generated when a player's forces achieve a certain number of victories, and can be used either to give a small amount of extra experience to all units trained in a city, or a very high level of experience to one unit. However, like other units, Great People of all kinds can be attacked and killed before the players can use them.
Buildings perform any number of different functions depending on building type; for example, early buildings such as granaries improve food storage to boost the city's growth and barracks produce better military units, while later buildings such as factories increase general production levels. There are also a number of unique buildings throughout the game. Most notable are World Wonders, which can be accessed through research nodes in the technology tree and construction through the worker unit. World Wonders provide advantages that are unique to each civilization, as they are limited to only one or two players.
Through buildings and specialists, each city also generates the "Culture" resource that contributes towards both the area upon which the city can influence for extracting resources and the overall civilization's cultural value. When two cities of different civilizations are adjacent to each other, the culture values of each city influences the space they can control; it is even possible that a city close to another civilization's city will join that civilization if their culture is strong enough. The high levels of culture gathering and attainment are also one of the default conditions that can be used to win the game.
Within the technology tree are technology nodes relating to government civics and state religions, each with their own pros and cons. The player has the option of selecting which set of government civics or state religions that the player wants their civilization to follow, but not all civics and religions can be encompassed.
Once the player has formally met another civilization, they can perform diplomacy at any time. For example, if the two civilizations are on friendly terms, the player can ask to trade units and/or technology for gold and vice versa, or request opening of national borders in order to freely explore in the other civilization's territories. The player can also use the diplomacy menu to request help in a war against a third civilization, or formally declare war on any civilization in which they engage diplomacy.
Religion plays a much more important role in Civilization IV than in the previous installments of the franchise. Impacting many of the game's key mechanics like government civics and diplomatic relations, the game's new religious system, according to Firaxis Games producer Barry Caudill, was added to increase gameplay depth over the entire game. 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).
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."
Discussion on the variable can be found in Proton issue #1209. Lower values maintain lower audio latency but crackling may still occur; higher values are more likely to eliminate crackling but allow for higher audio latency.
The scope of the game ranges from around 4000 BC to 2050 AD. Civilizations gradually advance in technology based on their own production of "research" and sometimes the work of Great People. Technologies range from writing and pottery through paper and gunpowder to genetic engineering and nuclear fusion. All technologies reveal new possibilities for a civilization and enable the chance to trade with other civilizations for military aid, gold, resources or other technologies. The concept of technological growth is based on a technology tree.
Another important concept in the game, not present in the earliest games in the series, is the growth of culture, which expands one's cultural borders and can also cause one's culture to infiltrate into another civilization's, sometimes causing a city to rebel against its current owner. Culture is increased through the creation of World Wonders (which may have bonus effects), constructing certain buildings in cities, and the spreading of a number of religions (see below).
The game can be won through conquest (conquering all other civilizations), domination (controlling a percentage of the world's land and population), the space race (being the first to construct a spaceship capable of colonizing Alpha Centauri), culture (increasing the cultural ratings of three different cities to "legendary" levels) and diplomacy (through votes in the United Nations). Finally, if the game's clock runs out (by default in the year 2050 AD), the nation with the highest composite score is declared the winner. 2b1af7f3a8