What confuses the eye (for me) is this: I assume that there is only one light source, and that it is at the bottom right hand side of the drawing. If that were the case, then that would be consistent with the convex shape. If you put the light source top left, then the walls would cast a shadow consistent with a concave shape, but - you just wouldn't see a light source work that way through solid rock which is why (my mind) 'prefers' to see it as standing proud with convex edges.

Try using a dark inner glow rather than a drop shadow so the shadow is all around the walls. That would make the eye think that the light source was inside the rooms which would make sense....I think!

Ravs