Thanks for the feedback! I hate level design and love coding new mechanics, so that's why every level is so different. Almost all of them have a new feature or something to learn about.
I'm glad to hear you thought some of the levels were challenging but fair. It is always a little tricky to determine these things because I ended up playing it so much that I got pretty good.
I enjoyed the music also! Each one (except Shinobi) was created for one of our previous games together and it was fun to put them in and listen to them again.