Wow, i love the art and the movement. The idea is really clever and fun, but i have a few critique points:
1. The UI quality is much worse than the rest of the graphics. Try to make it better next time by e.g. using a different font than the standard one. Same for the background of the level.
2. Maybe have a tutorial in the beginning. I got it after a few seconds, but i didn´t know how to undo something until i read the comments.
3. Make the undo button something more convenient than backspace, because if you use a mouse, you have to get your right hand off of it. Maybe space or something would have been better.
But still cool, that you managed to do this in such a short time. Also great how you intodruced the mechanics step by step. Would be nice if you could rate my game "Square Pair" too. :)