Galaga

Galaga is a single-screen shoot-em-up in which the player controls a 'Fighter' spaceship and must defend the home planet against the on-coming hordes of alien invaders called "Galagans". The Fighter can only move left and right along the bottom of the screen. Galagans fly onto the screen in a variety of formations before forming troop lines at the top of the screen. Once all troop lines are formed the Galagans separate and start attacking the player's Fighter in ones, twos and threes. The top-line Boss Galaga need to be shot twice before they are destroyed.