Thanks, the 3D quality of this is helped by the use of really dark shadows and as smooth as possible transitions. The choice of using a pure black shadow was done partly as a way of dealing with some of the difficulties that I was running into in imagining what specific tone different objects should be when seen in light or shadow. Specifically I was aware of the way some 3D games, like Abe's Oddysee or Doom 3 got away so well with just using black for the shadows.
but the top looks unfinished...