Very good considering the simplicity, but i noticed a flaw - you progress too often, I feel.
it seems that you kill a handful of enemies, then level up, whereas I would rather kill lots of enemies per level, then select multiple upgrades at the end of bigger levels. perhaps that is more preference than anything

and the gun does overheat a bit quickly initially, but this problem is somewhat phased out upon getting upgrades later on - only somewhat though I feel.

also I think 360 degree shooting should be implemented

however, I feel there is a good, diverse variety of enemies and bosses. They all have unique features such as different ways they behave and shoot. I think that's imperative for a game like this otherwise it gets boring, which yours doesn't

overall 6/10, good attempt :)

Unfortunately I uploaded a .rar without seeing it only accepts .zip

that is now fixed, however