This game is a gem requiring a fair share of mental gymnastics and patience. Thanks for not including a pesky timer. However, there have been occasions where I have wished for a hint system. Perhaps one where, once a puzzle is solved, players are given a free "hint coin". This gives players the opportunity to rack up some hints in the earlier rounds to be used in the later, more difficult ones. Also, you could always put a limit on the number of hints awarded (e.g.) maximum of 25; hints given after solving two puzzles, etc.
The interface, while a bit spartan, could use a bit sprucing up. Also, "sparkle" flourishes (for example) can be added animation after a puzzle is completed. These are some initial thoughts. Overall, a very good game.