More concept thumbnails.

With these newest thumbnails, I considered using references from other cave pictures and city-scapes to get a sense of scale and depth just right. What I noticed with buildings far off in the distance but with a grand scale is that the lower half of them is faded out due to distance. Using such props as windows, then shrinking them down also seems to work when the building is closer to the camera.
Tip: Use pieces of scenery in the foreground that are familiar to us as being large, then draw a much larger building behind to denote a sense of scale.
I also considered integrating a variety of cathedral architecture along with my Greek for a bit of variety.