0 A.D.

0 A.D. is a free and open-source, cross-platform real-time strategy game under development by Wildfire Games.
 It is a historical war and economy game focusing on the years between 500 B.C. and 1 B.C. for the first part, and a planned second part for the years 1 A.D. to 500 A.D.
 The game is cross-platform, playable on Windows, OS X, Linux, and OpenBSD. The game aims to be entirely free and open-source, using the GPLv2+ license for the game engine and CC BY-SA for the game art.
0 A.D. originally began as a comprehensive total conversion mod concept for Age of Empires II: The Age of Kings in June 2001. With limited design capabilities, the team soon turned to trying to create a full independent game based on their ideas.
 The game has been in development since 2000, with actual work starting in 2003.” source: wikipedia

You can download the game from the project webpage, sourceforge, github or install it directly from your distribution repository through package manager.