It's really satisfying when you figure out the answer, but it's pretty hard to get to that point. Lvl 4 has me completely stumped, to be honest! I think this would have really benefitted from just a couple of easier levels to get the hang of the puzzle concepts. I think the art is wonderful but the gameplay would have been just as fun with simpler art, which may have given you a bit more time to allocate to level design.
Great job, guys!