Wow, this is awesome.
Great idea, great execution, and the music was so good :o
I don't know if monsters being stuck in pistons (and being able to pull them out of pistons) was intentional, but it works out pretty well in the end.
I had no issues with the gameplay/design, and the art/music/sounds were excellently done. Hearing more instruments added whenever I joined monsters was so satisfying!
Great job!