Tower defense finds its roots in the strategy classic Rampart, an arcade game from 1990. The game involved defending a castle by placing cannons, and making repairs between several rounds of attacks.[6][7] By the new millennium, tower defense games began to appear in user-created maps for StarCraft, Age of Empires II, and WarCraft III.
http://en.wikipedia.org/wiki/Tower_defense
Tower defense finds its roots in the strategy classic Rampart, an arcade game from 1990. The game involved defending a castle by placing cannons, and making repairs between several rounds of attacks.[6][7] By the new millennium, tower defense games began to appear in user-created maps for StarCraft, Age of Empires II, and WarCraft III.