Subterranea

Subterranea is Imagic's entry into the Defender category. Subterranea's gameplay is broken into two types of screens. The first is a side view dogfight between your ship and some hapless enemies that are trying to home in on it. Things heat up in the second (and far more common) type of screen, which consists of Defender-style action minus the humanoids. You're required to shoot a specific number of aliens, then pass through a gate to the next stage.