You said it was not so polished, but this is a lot more polished than many games in the jam! A really great job! The art is very simple, but all the effects make it really cool. The music fits the artstyle really well as well. The subtle screenshakes, soundeffects and beautiful explosions make the game feel great to play. All-in-all amazing job on the atmosphere and the visuals! :D
I guess if I had to give some criticism it has to be about the gameplay. In principle it is really good and clear. The two enemy types are clearly different and interesting. The health spawns were also very clear, but I think those were a bit too powerful. They fully reset your health, which for me meant that I could not really lose. It was always possible to at least avoid most of the enemies, but the damage I unavoidably did take did not matter because there would always be a health spawn in time. Perhaps the spawn chance/time of those health things could be reduced over time? In that way the difficulty scales a bit more. Just an idea :)
What I really liked was that there was a red region before an enemy actually spawned. This made it so that you could not walk straight into an enemy that spawned in front of your face, but you could anticipate what would happen. A little problem was that at some points half the screen was red, so it became a bit difficult to see what was an actual enemy and what was a spawn region.
I am sorry if this is a bit too much criticism. I just really liked the game so I guess I am just a bit enthousiastic, haha. Great submission! Especially if this is your first real game. I am truely impressed :)