This is a fun mini power-fantasy defense game!
I love the low-key graphics and audio - good sense of style there. Simple gameplay loop, very direct type of control. You can probably make some money off this as a mobile game. Steady supply of upgrades provide steady progression, and I thought the upgrade rarities were very cool, though maybe they also have a drawback in that they end up being the only choice in that upgrade cycle since they are probably optimal.
What I think detracted from the game a bit was the incredibly repetitive music. I think the effects of Q and E keys could also be telegraphed to the player a bit more clearly.
Overall a very good game, well done!