As some have said before, the difficulty is the most annoying thing. I needed about 30 tries to get the boss down. The Only way is to kill all the Skeletons without being hit once, while being lucky that many of them drop orbs. In addition every time you die, you have to climb over that hill again, which also was annoying.
The Game was a bit short, and finding the crystal-part on the right side was too much of "run right and down" cause you didn't even have to kill the enemys on the way. Some sort of location-device and perhaps a bigger map (including map-screen) would've been nice.
Anyways, more enemys, more depth for the story and some Stage-bosses that aren't that hard to beat would (in my eyes) improve the game. Also: Why not make this a series?