I'm glad you enjoyed it! I appreciate the feedback.
The levels do not have checkpoints intentionally, because in the final game there will be a running timer (I took it out for the demo) so that you can get better at the levels and bring your time down. I do agree that some of the harder levels can be frustrating, but none of the levels take more than a minute to beat so I hope the players will learn how to quickly get through them over repeated tries.