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.
Like Defender, aliens come in all shapes and sizes, and have varying attack patterns. Some of them shoot at you, some bounce, but all the action is a far cry faster than Defender. Once all the aliens are destroyed, you must pass through an electro gate that zaps in certain patterns--failing this forces you to repeat the current level without scoring any points. After at least three Defender style screens, it's back to the dogfight. Subterranea awards extra lives not by scoring, but by completing stages without being hit. while Subterranea borrows concepts from other games and doesn't appear to be very original, there's just something about it that keeps you coming back for yet another game.
31st December 1983