Had motivation to play to around wave 15 or so. At least until that point the game felt too easy, but maybe harder times were still ahead? It was quite easy to cheese enemies by phasing through walls.
I would have appreciated that I could just hold my mouse down. Clicking to shoot does not really serve any purpose other than tire my hands.
Upgrades were simple, but effective. It took me a while to notice the transparent shop button though.
But overall, a simple but solid entry!