Imagine Robotron or Geometry Wars with crystal collecting and a massive variety of enemies, and you've got Crystal Quest. This game is awfully derivative, but still somehow manages to be totally engrossing due to its surprisingly intense difficulty. Crystal Quest doesn't hit you with a serious challenge right out of the gate, though. It has a gradual learning curve that keeps the early levels easy, but eventually ramps up to the kind of arcade chaos that will fuel a personal vendetta.
Over the course of the 60 waves of this gem-nabbing, baddie-blasting riot, you'll see so many different kinds of enemies (each with their own annoying behavior) it'll be difficult to keep them all straight. Some just meander around the screen fairly harmlessly, others hunt you down in a kamikaze charge and still others glide across the board firing ricocheting bullets at you from afar.