Alien Defender is a single screen shooter where the idea is to move around the screen in your spacecraft blasting as many aliens as you can trying to have more kills than the aliens. When you blast an alien your kill score increases but if you are touched then the aliens kill score increases. Before the start of the game you can select the speed from 1 - 999 with one being the fastest and as you play the speed increases. Moving off the edge of the screen makes you appear on the opposite side.