I really enjoyed this, and am hugely looking forward to Cassette Beasts. The only real problem I have is how heavy this seems to be for a Game Boy-styled game. It hiccups when there's a lot of monsters on screen or a few glitch effects happening at once, and sometimes the end boss slows to a ridiculous crawl. My laptop isn't amazing but it exceeds the stated requirements.
A minor improvement I would make is to allow for map notes, so you can come back to a tile/dungeon when you've got the upgrade you need. It's difficult to remember what you need to go back for.