Nice game, I like the ammo concept!
Here is some of my experience when playing this game
1. when I first lookup on the first, I thought this is a "top-down game" because of the background
2. The UI stats (the one in the left corner) not updated after I upgrade it when clearing the level
3. The stats that are on the upgrade panel (the panel that shows up between level) is all the same as before when nothing at all changed until you click upgrade on it
4. Maybe prevent the player to skip level when the health is 1
That's all, hope this will help you in any way possible