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).