Quite a nice puzzle game. The design is clean and the controls feel good. I do think there are too many puzzles before any challenge really starts. (I would say it only began from puzzle 24) But since they are quick, it isn't a deal breaker. There are a few weirdly trivial puzzles right after difficult ones. Number 51 and 54 are examples.
While the change in background music is a good idea, I found the difference between the songs jarring, as it had a huge effect on the vibe of the experience.