Defender is a two-dimensional side-scrolling shooting game set on the surface of an unnamed planet. The player controls a space ship as it navigates the terrain, flying either to the left or right. A joystick controls the ship's elevation, and five buttons control its horizontal direction and weapons. The object is to destroy alien invaders, while protecting astronauts on the landscape from abduction. Humans that are successfully abducted return as mutants that attack the ship. Defeating the aliens allows the player to progress to the next level.