I liked this mix of a shooter with a little bit of puzzles. Nice art style too, and the difficulty ramps up just as it's supposed to (except maybe the last level is a bit too hard, although it might be fine since it's the end).
I think you could add some visual feedback when activating the doors and stuff because sometimes it's not obvious. And maybe some wires connecting the buttons with the doors, because in the first part of level 4 I didn't get that you have to place the die on the button in order to proceed.
Also, having some music would improve the experience.
Aside from that, I enjoyed this game, nicely done!