Great game. I was buffled when I found out that you can crush enemies with boulders and that the upward gravity affects the shooting ability. Wonderful details.
The checkpoints were a bit unforgiving, I had to stop when i fell from the elevator, because it took too long to get back up. Also the controls felt a bit strange, but that might be because of my QWERZ keyboard layout. Key remapping would have helped with that.
The graphics and sound were nice and just the fact that there are so many and so huge levels is impressive.
Well done! I hope I manage to play it again to make it to the end.