Eh, to be perfectly honest, this didn't feel like you put a lot of work into it. Sorry! The premise was promising, but the simplistic visuals kind of killed it for me. The "marble" didn't look like marble, the sewers looked more like a lake with some stuff rising from the water here and there, and so on. (I mean, if you wanted filth, you could've used brown/red water and brown/red waterfalls, you know?) The plot doesn't come together well, either: it's very easy to mistake [spoilers]the instant death screens at the end for an ending. Works perfectly as sarcasm. I only realized the level had a proper ending after I checked it with the level editor. And the proper ending did not work well, since well, what the hell is Skyhome?[/spoiler]
I had quite some trouble with some of the challenges, so I'd rate this hard rather than medium. I don't know if you made some things easy to miss
on purpose (i.e.
the double jump powerup - you can get to the marble area without it - and the air that takes you to Skyhome
) - if you did, well, they don't quite work since finding them isn't much of a challenge and, because of the poor visuals, not interesting/not exploration.