The game mechanics are simple, I guess... I have no complaints about the core mechanic. The background really looks nice. I like the urban feel you're using with this.
I cannot help but bemoan the lack of music or ambience apart from the background. It's so quiet; I would have hoped even for the faintest traffic noise, but not even that was present. It's too austere. What the game also doesn't tell us is that even touching a box by its side makes you automatically lose, despite not being out of bounds. It's a bit of a nasty shock.
The other thing is that I get that you're handling a box monster. At least *make* it look remotely evil... Or maybe, give it some character. Make it a pretty, pretty box that's actually evil inside. Surely it's not that far-fetched? You've got *some* artistic skill, at least from what I see with the BG. It looks so good; I really, really wish you could make your imagination run wild with this.