Galaga is fixed shooter arcade game developed by Namco in 1981.  Galaga comprises of a spaceship in the lower portion of the screen, and at the beginning of each stage, enemy aliens fly in to shooting area where they form into rows.  During the game, the shooter spaceship must shoot all the aliens in formation and once all the aliens are destroyed the player advances to the next stage.   

Points are based on destroying the alien enemies.  The fighter ship has the ability to move left and right in an effort to dodge and destroy the aliens.  Occasionally an enemy will try to capture a fighter ship with a tracker beam, and if this occurs the fighter ship is held by enemies until another fighter ship frees the captive.   If released, the player has the ability to shoot with two ships simultaneously.  The game is over when all figher ships are destroyed or captured.

A Challenging Stage occurs after every three levels where pattern formations of aliens fly onto the screen.