Alien Abduction

Yes, the basics of the gameplay derive from Defender--it's a sidescrolling shmup in which you can 'flip' your ship to move and fire either right or left, and enemies approach from both sides of the screen. But Alien Abduction features trippy late-80s graphics, excellent sound and music, 30 levels, and 3 gameplay modes.

This is also not an easy game to play; a single hit can kill you, and it was obviously developed by serious shmup fans who like their gameplay challenging (if you want something less intense, try Gamma Bros). But if you're a serious shmup fan, or having been playing the hell out of Geometry Wars and are interested in something of the same intensity with different gameplay (and better graphics), you must check Alien Abduction out.