Smart mechanic and gameplay loop!
Art style is nice and charming!
It starts to be very intensive when all is left is just a melee block and there are couple big enemies running around!
It seems that money is a pretty random thing to appear. I feels a little bit unfair. In one run I had an apportunity to buy one shield block at 4th level. It might be better to have some minimal amount of money per level + extra for killing enemies so that player can play with upgrades earlier.
Overall is very cool concept and strong submission! Will recommend to play!