Awesome puzzle design! it has the assumption-revelation mechanics of normal "pushing boxes" games, but this one has an extra dimension! which makes so much more interesting. the difficulty curve was almost perfect, i would say the jump from 6 to 7 is a bit steep, but the rest is great.
One thing that did make me mess up the puzzles a couple of times was how the game detects that you are working on another face. I would sometimes quickly pan over to a new face and because i was trying to do it quickly i actually ended up moving the box on the previous face before the game switched over to the new one. One possible idea would be to add a slider for the 'angle' of detection or something like that (this is of course assuming you want to expand the game).
Before writing this i was gonna suggest adding a "step-back" feature, but after thinking about it, it may actually mess-up your difficulty curve, so now i'm not sure.
Again, great game!