A city is the basic unit of a civilization, and in many ways its most valuable possession. Cities do most of the advancement in a civilization, generating Image:20xFood5.png Food, Image:20xProduction5.png Production, Image:20xGold5.png Gold, Image:20xScience5.png Science and Image:20xCulture5.png Culture, as well as Image:20xfaith5.png Faith (in Gods & Kings) and Image:20xTourism5.png Tourism (in Brave New World) for an empire. Cities also have the ability to produce buildings and units, and build Wonders. All tiles with cities on them have a road, even if The Wheel hasn't been discovered yet.