Galaxian

Galaxian is a shoot 'em up in which the player is at the bottom of the screen, with an arrangement of aliens at the top. The player moves left and right to aim at an alien, then shoots a bullet up the screen, and the alien it hits is killed. Instead of the normal 4 alien types in Galaxian, the Acorn Atom version only has 2, but retains that those higher up the screen score more points when destroyed. At set intervals, an enemy will move down the screen towards you, which is a moment of high danger. Clear a wave and another is generated.