Very good game. I think you shold balance the physics. Lv2 I was able to go to NEXT round even without de box in the truck. Lv 3 (the one with a lot of stairs - sorry about my english, I dont know how to call that) I had to try more than 10 times to be able to go to the next level.
So, I think you should rethink and balance the game physics, but it is a very good puzzle idea.