Very nice! The concept of linked boxes and the way you indicated it was clever. I was particularly impressed with how you designed the levels to naturally teach the mechanics. I had a couple of "aha!" moments while playing :)
The one criticism I'd give is that the later, bigger levels weren't as fun or interesting as the tight, early levels. By level 11 I was just ignoring the boxes and running through the fireballs because there aren't any consequences for dying.