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.