Well, looks like I have lot of things to consider before starting. Yeah, I think vertical is better for an underground city too.

Steel General, your map is pretty good but my ideia was a little different. However I think i'll have to do like you did, maybe draw three cities (3 different levels) and then another print where I would show the location of the levels, and then but everything together o.o .

"Ancient undergounr cities" I haven't tried yet, only "undergound cities" or "undergound city artwork/print/draw" but what pop up is some tunnel imagens of real cities, and usefful stuff '-', I'll try your ideia.