The most influential video game of all time. A single player moves an armed 'base' to the left or right along the bottom of the screen and fires at the endless waves of aliens that tirelessly parade across the screen towards earth. There are 4 buildings (shields) at the bottom of the screen behind which the player can hide, but these will eventually be destroyed by enemy missiles or by direct contact with the invaders themselves. Player shots will also destroy shields. The descent of the aliens accelerates as they are eliminated, making them more difficult to hit. A flying saucer will fly across the top of the screen at regular intervals and can be fired for extra points.