Mattastic, that looks great! Do you really need a ceiling? With the perspective you're using, you wouldn't be able to see it (except for the front-facing edge similar to those you can see on the walls), unless you made it possible to see a much higher point. I personally think it's probably going to look better the way it is, without the ceiling.
The style and the shading are really nice. I also really like the way the shadow looks. The only problem is the lighting. Since the walls get lighter toward the bottom, the floor getting darker further back kind of clashes with this (what's making the walls just as bright in the back as in the front?). There isn't a light source behind the character, so, if anything, the shadow should be facing the other way. Honestly, though, I think I'm only noticing this stuff because I'm looking for it. The depth looks nice, even if it's not 100% realistic, so I'm not sure it's necessary to change it.