I started playing with fairly low expectations but I actually had quite a bit of fun. It is fairly short (only about 10 levels?) and some of the levels were possibly trivially easy - although this is not such a bad thing. When a player has worked hard on a level, sometimes its a nice reward to be presented with the next level being very easy.

If you were to take this further, then it really needs a title page with level select.

I enjoyed this. Some more polish would make the game really shine, and even without that, this is a nice twist on the Sokoban formula.

Good job on implementing undo.

If you take it further, maybe some scoring mechanism: e.g. fewest moves to solve the level, for example. That would ensure that there is a challenge even with undo, and provide some replayability.