Colony Defense is a tower defense game. Player takes place of GalCon war academy graduate with mission of protecting human population on distant planets of year 5736.

The game field has a shape of sphere, with several enemy landing bases and peaceful citizen colonies connected by roads. Various gun towers should be built (keeping the distance between each other) across the paths in order to prevent alien invasion, but they also might be sold (useful when enemy gets immune to certain damage type) or upgraded. The only resource are credits, for which towers are bought. When thing get really bad, it's possible to strike once in a while with orbital ion cannon, dealing large area effect damage.

After finishing the mission, player receives one talent point and another one in case of zero civilian population loses. They are spent for upgrading certain aspects of alter ego, like cheaper building price or higher damage.