Really awesome little rogue-lite game! I had a lot of fun with it!
Each time, I died, I wanted to continue again until I beat your game, the upgrade menu is really clever. And I found that it is well balanced overall.
The only critiscim that I can make: it is for the selection of Spells that I found it quite slow in order to change spell. So, for me if you extend this game in a bigger one, I think that you can improve a lot the Spell selection, for example by assigning a unique key for each spell (like 1, 2, 3, etc. of the keyboard).