Defence Force is a clone of the game Defender: a side scrolling shoot'em up where you have to defend humans.

Humanity has been almost totally destroyed. You are a pilot of the Defence Force and you have to kill waves of aliens coming from both sides of the screen. You will meet 16 different types of aliens, some moving only in one direction, others trying to crash in to your ship, others dividing in smaller ships when shot, and some are even invisible. Radar shows you where enemies (even invisible ones) and humans are located on the level.

The Vulturae type of alien ships should be you primary target as they try to abduct humans to transform them into Assassins (which will try to crash in your ship). For any alien wave, if there is a time where there are no humans left, you will be warped into the Astral Plains, where all remaining enemies are turned into Wraith, a fast skull-looking ship which tries to bump your ship away. To help you to survive, you can activate shields, the screen is then blue and you are invincible.. for a few seconds (6 seconds on the first wave, only 1 second in the more difficult high levels).

The player uses the cursor keys to control the ship. There is no fire button: each time you press the move key, the ship shoots a laser beam.