Very neat idea and great progression: I always felt the that the previous levels was a sufficient foundation for the next. By the way, is the yellow character in the picture a planned hero or an enemy?
Criticism: I get that you want to give the heroes personalities. This adds to the enjoyment. However, I like to discover mechanics on my own, so I would prefer if they didn't spoil every mechanic right away (of course you have to convey the controls somehow). Maybe they could give hints after a certain amount of time has passed.
Undo and redo would be greatly appreciated, though I realize that takes some work to implement.
Thanks for the experience and I hope that you can make a full game from this!