0 A.D.

0 A.D. is a free and open-source real-time strategy game which puts players in a classical antiquity setting. Each civilization is unique both visually and mechanically, and unlike most other strategy games, combat units can contribute to the civilization's economy. The map is divided in player-controlled territories much like in Rise of Nations, and these territories can only be expanded by constructing buildings next to them. Also, much like in the Age of Empires series, players unlock new technologies and units after they reach a new phase (analogous to ages). While there is no singleplayer campaign, the game can be played in offline skirmishes against the AI or in online multiplayer.