Cool little game! The simple graphics work well, and the SFX and music choices are great too.
It's pretty challenging! I made it to level 4 before throwing in the towel.
I'm not sure how I feel about the insta-death for falling. On the one hand, since the entire game revolves around platform building, maybe it's necessary. But I feel like maybe just doing large damage and sending you back to the start would be less frustrating. Another option would be to go full hardcore, and have the bullets instakill too. Maybe they'd appear more slowly as levels are passed.
Overall, it's a good concept and well executed! Nice work.
(Found one small bug while typing this long message; the music appears to stop if you stay in a level too long. It restarted when I jumped to my doom and the level refreshed.)