This is pretty much the absolute bare minimum necessary to have a functional sewer level. Can't you add some decorations and gates and so forth?
Also, those shadows by the walls make no sense. There isn't light shining down on the walls from above to create the shadows - there's a ceiling. If you want to have any lighting effects, you should either have wall lamps, or cover everything in shadow that's outside of a circle around the hero, as though he is holding a lantern that's the only main source.