General game information
More information on ModDB.
The Design Document - This is the main over-view design document for the game. It should give you a good insight to the game workings and big-picture.
0 A.D. Gameplay Manual - details on how to run the game.
0 A.D. Modding Guide - overview of modifying the game. What tools you need, where the files are located, etc.
Forums and Community
#0ad-dev on QuakeNet IRC for developer discussions.
Human Template (RAR'd PSD)