Shoot the aliens.
The player controls a laser cannon by moving it horizontally across the bottom of the screen and firing at descending aliens. The aim is to defeat four rows of aliens that move horizontally back and forth across the screen as they advance toward the bottom of the screen. The player defeats an alien, and earns points, by shooting it with the laser cannon. As more aliens are defeated, the aliens' movement and the game's music both speed up. Defeating the aliens brings another wave that is more difficult, a loop which can continue without end.
