The two different layers to the maps were well designed and the ability to preview the alternate dimension was a really nice touch. I loved the music and the fact that it changed along with the dimension. It sets a distinct mood in each dimension really well.
I found the controls unintuitive at first because I badly wanted to press W or Up to jump instead of spacebar. This is tricky because then I found that you use W/Up to climb ladders. I think it would feel more natural if you allowed W/Up to jump and perhaps when on a ladder, holding the key would make the player latch on to the ladder to climb it. Just my thoughts.