This game is based on the classic Space Invaders game.
You control a gun turret at the bottom of the screen, and must fight off wave after wave of alien invaders. Of course they can shoot back at you and kill you by touching you. The game levels get progressively more difficult as you go along, with the aliens increasing in numbers and speed, and acquiring better, faster bullets.
This program requires Mac OS X 10.4 but may also run on 10.3.x.