The levels started off being very intuitive, but then eventually got less so, to the point where the last level basically requires you to use the hints in order to make it work. I feel that that just isn't good design - it should be intuitive like (almost) all the other puzzles. You can make it harder by making it take longer to figure out, but it shouldn't be mandatory to click on the hints. Otherwise a great game. Level 16 is a bit spooky, haha.