Aliens is a single-screen shooter which features a common setup: aliens attack an unnamed planet which is represented by green hills on the bottom part of the screen. On these hills there are fortifications which fire at the attacker. However, in this game the player controls the alien ship instead of the defenders. The ship can be freely moved within the screen and shoot bullets straight down. The goal of each level is destroying all defenders. The player starts out with five lives which are lost when the ship is hit by a bullet or crashes into the hills. After losing a life, the ship instantly respawns without losing progress in the current level. The game ends when all lives are depleted, but an additional one is gained when progressing to the next level. The game simulates inertia and sometimes extras fall from the sky which can be picked up before they reach the ground. The level layout (the look of the hills and the position of the defenders) varies at each start.