This game is harder than it looks! The fact that the enemies and bullets can pass through the box and wall makes it more like a puzzle game, so it's great that you can retry so quickly. I got to the level with the double walls, but I gave up, because it was too hard for me; however, it is quite fun to play for such a seemingly simple concept. The box disappeared on the double wall level at one point, but that's okay, since you can just replay the level quickly.
It's a little hard to get the box to go in the direction that I want it. Maybe it has a default push to the right direction, so trying to push the box up was difficult in the double wall level.