0 A.D. (pronounced: "Zero ey dee") is a free, open-source video game currently developed by Wildfire Games. The game is focused on historical war and economy placed somewhere in between the years 500 B.C. and A.D. 500. The game is open-source and cross-platform. The development started in 2000, but only later in 2003 the actual work has begun. The developers do not have an official release date for the final, stable version of 0 A.D.
In 0 A.D. you control a civilization, starting wth only a Civic Centre and a few basic units to help get you started. The main goal is to develop an army and defeat the other players, very similar to most historic real-time strategy games. There is also an economy system, you have to gather resources by assigning your units to different jobs like: chopping trees, farming, fishing, stone and metal mining, etc. The player must balance each of these jobs in order to gather resources efficiently depending on his needs. The game supports both land and naval warfare.