Where do even start...
First of all, not bad, not bad at all. I do like the physics you made were, has the right feeling to it.
Second, one of the most creative uses of the art assets by far, well done.
Now for the critique.
1) The UI smears when you move too fast - that's a big no-no.
2) The UI has the same color as the ground (white), making it impossible to read unless you actively look up.
3) Enemies are damn unbalanced.
a) The enemies respawn waaaay to fast. Already on stage 1 I have to just stay and kill the very first mob all the time, which makes progressing kinda impossible. Ofc I can just ignore the mobs and run around collecting the upgrades, but that also isn't a viable strategy since I get swarmed then.
b) This is inconsistent, but they also eat too much health (sometimes they don't, idk what is going on). Pretty much one touch and you are dead.
c) Either the monsters can spawn on top of me and just insta-kill me, or I get randomly killed for no reason. Either way it doesn't feel good.
d) The hp scaling for the enemies is also too high.
4) It would be nice if the enemies could forget about you, for instance if you get out of sight they could get get to the last spot they saw you and then stop, and not chase you through the entire map, flying over the walls.
5) Yeap, some sounds and music would be nice.
6) Physics bug(?) - if you get close to a platform that you can jump on and try to jump while holding forward, you can't really jump onto the platform, but get kinda glitch-clipped and sent back where you were.
7) The intro/tutorial level should be optional, gets boring really fast. You can also accidentally soft-lock yourself there if you just run past the mobs and jump to the rocket without killing those.
8) Add some nice animations for the gun firing.