Sid Meier's Civilization IV

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 34 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.

The game engine for Civilization IV was built entirely from scratch, with some help from NDL's Gamebryo engine. This decision resulted in a full 3D immersion of the game, which was the first in the series, and which allowed easier readability and smoother, more in-depth zooming capabilities. Some of the quotes relating to the technology tree and narration for the game were provided by Leonard Nimoy. The soundtrack for the game features compositions of Medieval, Renaissance, and Baroque origin, and design for the title music was influenced by The Lion King.

Composer Christopher Tin wrote the opening theme song "Baba Yetu" (Swahili "Our Father"), a rendition of the Lord's Prayer, which was performed by Stanford University's Talisman A Cappella. The song, when rereleased, became the first piece of video game music to be nominated for and to win a Grammy Award.

Civilization IV was a commercial success, and sold more than 1 million units by mid-March 2006. By that time, it had held a top-10 position on every weekly computer game sales chart released by The NPD Group since the game's launch. NPD declared Civilization IV the 11th-best-selling computer game of 2005, and it rose to ninth place on the firm's annual computer game sales chart for the following year. It returned to NPD's year-end top 20 in 2008 with a 13th-place finish.

The game also received a "Silver" sales award from the Entertainment and Leisure Software Publishers Association (ELSPA), indicating sales of at least 100,000 copies in the United Kingdom. According to Take-Two Interactive, sales of Civilization IV surpassed 3 million units by March 2008.