Not very innovative, but cool anyway. I'm also cool with the ammo limitation since you get an upgrade point if you're defeated anyways. That, however, makes the game run... unchallenging very fast. Adding to the unchallenging part, there is also the case of shuriken and arrows - very unbalanced when compared to the boulders late in the game.
The game is, all considered, fun to play, though. :D