Above anything else, I think this game is in need of an undo button. The first few levels were pretty easy, but it quickly became overwhelming and I only managed to get to level 18 before moving on to something else. Having an immediate undo would have made experimentation a lot more friendly, and probably would have made those later levels (especially the ones with two characters) less frustrating.
Having said that, this is a really great little puzzle game and I look forward to seeing what you do with it!
The music was nice too!