satoshi: It's like a shmup meets isometric platformer (a la Sonic 3D Blast, although it plays much smoother, in my opinion). If you shoot enemies to death, they turn into animals (which you need to collect), if you smash them with your hammer, they blow up, which can cause chain reactions that make enemies drop gems (which you also need to collect).
You also have a little egg that follows you around that will turn into different creatures depending on how you play with branching paths. It's pretty challenging, but it may have just been due to screwing around with the options and setting it to hard by accident. Graphics are freaking beautiful, and the boss battles are epic.