First off, congratulations on your first game, and either way this is a win (it's way better than my first many games). It's a really good concept and I love the 3d pixel art look. Unfortunately I got stuck on that black cube in level 3. I tried walking around it and shooting it, but it kept killing me, so I wasn't able to finish the game.
The only other thing that I noticed was that in a couple placed the ground got a bit warped, and I wasn't sure if that was intentional or a glitch, but not a big issue. Great work, and good luck with future games!