Bit too hard
This was really nice game, I liked the upgrade system, pixel art, enemies were quite diverse.
But difficulty curve is bizzare - I passed the first level without any trouble, then level two was pain in the ass - few foes that nearly cannot be destroyed and almost no upgrades. Sure, it can be passed, but it there's no fun in it.
I would love, if you made it a bit RPG style; I mean that even if you lose a level, you keep some of the upgrades, so you have more chance next time.
Also continue option would be nice.