Space Invaders

Space Invaders is a fixed shooter in which the player moves a laser cannon , referred to as a "Laser Base", horizontally across the bottom of the screen and fires at a group of the titular alien invaders overhead. The invaders begin as five rows of eleven—the amount differs in some versions—that move left and right as a group, advancing on the shooter by shifting downward each time they reach a screen edge. The goal is to eliminate all the invaders by shooting them. Regardless of how many game lives remaining the player has, the game ends if the aliens reach the bottom of the screen. The aliens attempt to destroy the player's cannon by firing projectiles. The laser cannon is partially protected by stationary defense bunkers that are gradually destroyed from above by the invaders; the bottom will be destroyed if the player fires when beneath one. As the invaders are defeated, their movement, as well as the accompanying music, speeds up. Defeating all the aliens brings another wave that starts lower, a loop that can continue endlessly. A special "mystery ship" will occasionally move across the top of the screen and award bonus points if destroyed.