Heya Mortis, this was a surprisingly nice short game. Although I pretty much died on Wave 4. There are good things here worth exploring,
I think you've got a solid gameplay loop going with the killing enemies, getting gold, getting upgrade kind of thing. It just needs more polish and balancing such as getting less gold, more chests (giving players choice).
In terms of possible improvements I think, much of them you yourself would know, but I'd point out some of the more jarring ones for me,
The way the camera zooms out and zooms in feels a bit weird at the moment. I think the scale of zooming out was too much, so much so that I always want to move to keep the screen zoomed out to see enemies from far away. If you adjust the camera better, I think it'd work alright.