0. A. D. free, open-source, cross-platform real-time strategy (RTS). Step back 500 years BC, and rewrite the history of Western civilization.
Strategy 0.A.D. was originally developed as a proprietary (private and commercial) product company WildfireGames, but then the code opened under GPL licensee and all the audiovisual material is licensed under a Creative Commons BY-SA. This made the game free and available for all popular OS.
The game has excellent 3D graphics, supports multiplayer games, and of course the game against artificial intelligence. The game covers 6 civilizations that existed in the range of from 500 BC 500 ad
System requirements: 1 GHz CPU, modern graphics adapter (GeForce 3 at minimum), 512 MB RAM or more.
You can download the game on the official website: http://wildfiregames.com/0ad/
In Ubuntu you can simply connect PPA repository: ppa:wfg/0ad (how to connect a ppa repository)
Then install the game via the software Center or with the command:
sudo apt-get update && sudo apt-get install 0ad