Illustration in Progress

Recently I've been working on an illustration that I intend to include in a gothic arch ATC swap. My original intent was to create a fictional setting, similar to an M.C. Escher drawing.
I started with this photo I took at the Library of Congress last spring for the basic layout of my illustration, to get my angles and foreshortening right.

So far I have an extreemly simplified line drawing. I think I'll leave it like this for the purpose of the ATC, more detail would only get lost and clutter such a small format. I havn't yet decided how much detail I should add, and should I add shadows, or just keep it outlines?