This concept is fantastic. I love minimalist rpg's (well all rpgs but a focus on the mechanics shows a lot of love for the systems and that's pretty sweet.) I had a ton of fun.
I did notice some bugs though. While playing and trying to fully upgrade what I now assume was the tutorial/starter class I was suddenly jumped to a class select screen between levels. I'm not entirely sure what triggered the change because I hadn't fully upgraded anything yet
The second attack speed upgrade in the rogue class disappeared at some point too before I could purchase anything in it. I didn't notice when exactly it disappeared but I can only assume the flag for unlocking the upgrade wasn't permanently flipped and since I'd fully upgraded the previous skill already I had no way of flipping it again
Also a rather minor issue but it was a bit unsatisfying that the final kill win or lose left coins on the screen that couldn't be collected. It would be nice if all coins left behind were auto collected when the level ended. maybe only on a win as a little extra reward? Just an idea.
I'm looking forward to see this grow.