Everything is so minimalist and beautiful; the excellent use of a controlled color palette and the overall aesthetic make it great.
I've been playing for quite some time and got to level 15, but my browser decided to close the tab for some reason when I pressed a key.
The character and enemy sprites are amazing.
I saw three types of enemies, and I really liked the progression decision: they increase in size and shrink when they take damage.
I felt like hitting enemies should give me some feedback, so they glow a little or something so I know I'm hitting them.
I also started to gain a lot of attack speed, so I was clicking the screen like crazy. This caused me to get an upgrade without reading it. It's always good to have a 1-second timer in these cases to protect against this happening ;)
The music is beautiful, but I can't say much more than that because it's not my strong suit.