Graphically, this game hits all the right notes for me. Minimalist but just oozing style. Lots of visual flair: squashy monsters, particle effects when you kill one, the lighting effect, the subtle camera move to point where the light is facing, the little isometric faux-3D spin of the protag, the screenshake. This is a really stylish game, and very polished visually. Plus, the monsters are really cute :)
The gameplay loop itself is very fun, but for me it really lagged in the endgame; after the first couple of minutes I never felt like I was in danger, so I could be pretty lazy because I could vaporize enemies really quickly. Unfortunately this led to a lot of waiting. The upgrades are really nicely chosen and they feel very powerful, but I think one thing I would suggest would be to scale the challenge with those upgrades; with each one, I think what you're chasing is that it feels powerful for a little bit so you get that sense of progression, but then the challenge grows to put the player back on edge. For me, even moreso than monster variety, I would really crank up the spawn rate (and/or the creeps' movement speed) over time (or a little while after each upgrade) so that even though you're getting stronger, you still have to really keep on the edge of your seat. I feel like if there had been a kill counter and things ratcheted up a bit more, I would definitely have kept coming back over and over again.
But again, the core loop here is really nice. In its current form, I played for around 25 minutes, but I feel like with a couple of small tweaks to ramp up the difficulty it could have sucked me in even more.
All-in-all, a really solid jam entry. Nice work :)