The game map is made up of square tiles on a grid. Each city, terrain improvement, and unit is located in a specific tile, and each tile can host any number of units, land tiles can contain a transportation improvement (road or railroad) and a land improvement (farm or mine) or a city. Cities must be built a minimum of two tiles away from each other (no two cities can be touching). Each tile is made of a particular type of terrain that determines, among other things, how much food, production, and trade it produces when "worked". A tile can only be worked if it is one of the 20 tiles surrounding a city, a tile can only be worked by one city at a time, and each city can only work a number of tiles equal to or less than its population.