Civilization IV is a turn-based game in which the player builds an empire from very limited initial resources. All standard full-length games begin in 4000 BC with a settler who builds a single city. From there, the player expands an empire while contending with rival nations, using the geography, developing infrastructure and encouraging scientific and cultural progress. By default, players can win the game by accomplishing one of five goals: conquering all other civilizations, controlling a supermajority of the world's land and population, being the first to land a sleeper ship in the Alpha Centauri star system, increasing the Culture ratings of three different cities to "legendary" levels, or by being declared "World Leader" by winning a popularity election through the United Nations. If the game's clock runs out (by default in the year 2050 AD) with none of these goals fulfilled by any nation, the nation with the highest score is declared the winner.

The game's first expansion, Warlords, was released on July 24, 2006, in North America and July 28, 2006, in the European Union. A second expansion, Beyond the Sword, was released worldwide between July 18 and July 30, 2007. A remake of Sid Meier's Colonization, based on a total conversion of the Civilization IV engine, Colonization, was released on September 23, 2008.

As of March 26, 2008, Civilization IV has sold 3 million copies, according to Take-Two Interactive. The game was re-released along with both of its expansions in 2007 in an edition entitled Sid Meier's Civilization IV: Complete; the American version of this compilation was released on May 12, 2009, in a DRM free package that contains the core game, the two expansions, and the 2008 Colonization remake.

Civilization IV is a turn-based 4X game, in which the player leads a civilization from a small tribe to conquering the world over several millennia. Conquest can be done in at least four ways: diplomatically being declared the world's leader, using military to defeat all other civilizations, becoming technologically superior to the other players, or to win through expansion of the civilization's culture across the world. Additionally, the game scores each civilization based on a number of factors, and victory can be had if the player's civilization score far exceeds any other, or if at the end of a limited-turn game, the player has the highest score. The game can be played as a single player facing against one or more computer-controlled opponents, or through online multiplayer games alongside computer opponents.

At the onset of a game, the player determines how the world map will be generated, either loading a pre-defined scenario, or specifying a number of parameters, including player counter, climate, and landmass type, for a randomly generated map. Players can then select one from 18 specific civilizations, or allow the game to select one for them randomly; these civilizations are loosely based on actual nations in mankind's history, and give the player a leader avatar, an initial set of technology, and unique units that that civilization can build. The player also can set the difficulty of the computer-controlled opponents prior to the game. When the game starts, the player and opponents are randomly located across the square grid map. Most of the map will remain dark to the player until they move units close enough to a space to see what is there, trade with other civilizations for their map, or acquire technology that reveals the map to them; further, the game uses a fog of war that unless the map space remains in sight of a player's units for that turn, the contents of that space are not shown. Each map space has a terrain type, such as plains, tundra, or desert, that affects what resources can be obtained from it, movement rate of units through it, and possible special resources that can be extracted from it.

Each turn, the player has the ability to move any units a number of spaces based on their movement rate and terrain, including attacking enemy units, instruct certain units to take specific actions, adjust the governing of each city they have settled, initiate diplomatic contacts, and review their current status. The player starts with a single military scouting unit and a settler unit, which is needed to found a city. As the player's civilization expands, they can found more cities and expand their military.

Combat is initiated by moving a military unit onto the space occupied by an opposing unit, including those stationed in cities. Combat is resolved based on the statistics of each unit along with random chance, making it possible for weaker units to defeat stronger ones. Defeated units are removed from the game; if an attacking unit removes the last defending military unit from a space, it will move to occupy that space, and if that space was a city, then the attacker will have captured that city. The player has the option of razing an occupied city or installing a new government and bringing that city into its civilization. Any number of units can be stacked onto a single space and move as a group if so assigned, though when attacking, the overall combat phase is resolved by one-on-one unit battles.

In addition to combat, military units can be set to fortify a specific space, perform sentry duties around a specific area, destroy enemy city improvements, or can be set to auto-explore the map.

Non-military units include settlers used to found cities; workers and work boats used to improve land and water spaces, respectively; spies which are invisible to others and can perform covert operations in opposing cities; and religious missionaries that can spread religion in a target city.

Throughout the game, the player may have a chance to generate a "Great Person", including Great Engineers (production), Great Scientists (technology), Great Artists (culture), Great Prophets (religion) and Great Merchants (monetary). These units, named after historical figures, can be used for one-time boosts in various ways. A Great General provides bonuses for units that it is grouped with in attacks, while a Great Scientist can be used to either discover a new technology immediately or construct a special building that produces a good amount of research points. Building certain wonders or discovering certain technologies will improve the chances of generating a Great Person. However, as units, they can be attacked and captured before their use.

Once a city is founded, it will extract food, material, and monetary resources from nearby spaces, the number based on the population size of the city. Food resources are used towards city growth, while material resources are used to construct either new units or buildings/facilities within the city that alter how the city operates or may have other benefits to the player's civilization. The game will automatically allocate which spaces are used and how the resources within a city as it grows, but the player is free to manage the city directly. This can be used to turn a part of the population into one of several specialized occupations, at the expense of having one less space that can be used for resources for each specialist. These specialists instead can generate additional research, culture, or money from the city. Additionally, one can simply assign these specialists as idle, helping to generate happiness for a city. This may be necessary to counter unhappiness generated by military units, a larger war, a lack of special resources, or the like; a city that is unhappy may fall into a brief period of rioting where no production will occur.

Each city can produce one military unit or one building at a time, the rate determined by the amount of material collected from surrounding spaces. After researching the right technologies, the player can "hurry" production using the civilization's monetary coffers to produce the unit the next turn. The player can queue up a number of units and buildings which are then produced sequentially as the previous one is finished. Optionally, the player can simply devote the city's resources towards producing research, culture, or money.

Buildings range across a number of functions; early buildings include granaries that help to boost the city's growth rate and barracks that provide initial experience bonuses to military units, while later in the game, buildings such as airports, drydocks, and factories can made. There are also a number of special buildings that can be constructed. One type are World Wonders, of which only one across all civilizations can be constructed. Constructing these typically take a good deal of time but also provide a great benefit to the player's city or civilization. There are also National Wonders, of which only one or two can be building within each civilization. The availability of buildings is tied to the player's progress on the technology tree.

As the player expands their technology tree, they will uncover special resources on the map. Some of these are necessary for building certain units or improvements such as horses for horseriders and chariots, and oil for machinery units like tanks. Others are considered luxury resources, like ivory, cotton, or wine, and can be used to improve city happiness or as trade goods in diplomacy. These resources cannot be accessed until the proper terrain improvement is made by a worker unit for that tile. For example, a corral is needed for obtaining horses, while a plantation is required to get cotton or wine. Other terrain improvements can be made on spaces that lack these special resources as to boost the space's production for the city that controls it: farming can improve food production, while mines can increase resource availability. Roads, and later railroads, help to increase movement rate and to create trade networks between the player's own cities as well as with opponent cities.

Through buildings and specialists, each city generates culture that contributes both towards the area that the city can influence and subsequently use for resources, and the overall civilization's cultural value. If there are two opposing cities near each other, the cultural values of each city will influence which space is controlled by which city; it is possible that a city close to an opponent's city will want to join the opponent's empire if the cultural influence is strong enough.

Tech tree of Civilization IV

Once a city has been founded the player can select a technology from the game's technology tree to research. Each technology requires a number of research units to reach before it is discovered, upon which its benefits are immediately seen by the civilization. Most technologies have pre-requisite technologies that must be discovered first - for example, the player must know bronze working before they can learn iron working. The player can select a future technology, with the game then queuing all intermediate technologies to reach that. Technology development is necessary to being able to build new types of units, buildings, and improvements. Ultimately at the end of the technology tree are technologies to develop a colony ship to reach Alpha Centauri; being the first to launch such a ship assures a technology victory.

A screenshot of Civics option menu in Civilization IV'.

Within the technology tree are both government civics and religion technologies. When one of these are discovered, the player has the option of then changing their current government civic or state religion to this new one; though offered immediately after the discovery, the change can later be done at any time. These civics and religions have certain benefits that affect the whole civilization. If the player opts to switch, their entire civilization will have one turn of anarchy while the government switches over.

Religion plays a larger role in the game as well. Each city is tracked as to what religions it has adopted; adoption can come from when the religion is first founded, or through the use of missionaries that can spread religion from one city to another. This can be both beneficial and detrimental depending on what civics and religion the player is currently using; for example a city that has the same religion as the state religion can be more productive, while non-state religions may increase unhappiness within a city. Such missionaries are not limited to the player's civilization and can spread the religion to opposing civilization cities.

Once a player has met another civilization, they can perform diplomacy at any time. If the two civilizations are on friendly terms, the player may wish to extend a trade, which can include a lump sum of money, a regularly allocated payment over time, units, luxury resources, technologies, or even cities. Optionally, the two may wish to simply share their current world maps or offer open borders that allow the other's units to cross their territory freely. The player can also request that the civilization go to war against a third civilization. On the other hand, warring civilization may attempt to negotiate a cease fire, or make strong-arm demands from the other civilization.