Space Invaders

Space Invaders is a fixed-screen shooter in which Earth is threatened by a formation of descending aliens. The player controls a laser cannon that moves horizontally along the bottom of the screen and must destroy the invaders before they reach the ground. The aliens march from side to side, descending one row whenever the formation reaches the edge of the screen. Several visually distinct types occupy the formation, with those in the higher rows awarding more points. The invaders periodically fire missiles downward, and both their movement and accompanying marching sounds accelerate as their numbers decrease, making the final few enemies increasingly difficult to hit. A mystery ship occasionally crosses the top of the screen and awards bonus points when destroyed.