Defender is a side-view, horizontally scrolling shooter set on the surface of an unnamed planet. The player controls a spaceship flying either to the left or right. A joystick controls the ship’s elevation, and five buttons control its horizontal direction and weapons. The player starts with three “smart bombs”, which destroy all visible enemies. As a last resort, the “hyperspace” button works as in Asteroids: the player’s ship reappears in a random—possibly unsafe—location. Players are allotted three ships at the start of the game; another ship and smart bomb are awarded every 10,000 points (adjustable per machine). Two players can alternate turns.

The object is to destroy all alien invaders, while protecting astronauts on the landscape from abduction. Landers pick up humans and attempt to carry them to the top of the screen at which point they turn into fast-moving mutants. A captured human can be freed by shooting the lander, then catching the human before it falls to its death, and dropping it off on the ground.

Defeating the aliens allows the player to progress to the next level. Failing to protect the astronauts, however, causes the planet to explode and the level to become populated with mutants. Surviving the waves of mutants results in the restoration of the planet. A ship is lost if it is hit by an enemy or its projectiles, or if a hyperspace jump goes wrong (as they randomly do). After exhausting all ships, the game ends.