Nice ideas on this; looking forward to seeing how you progress on this. One thing you might try to show the difference in distances between the top of the plateau and the bottom is to use the same shapes of buildings in both places, but make the ones on the valley floor much smaller, kind of a visual trick. The viewer will see the same shape but notice the one on the ground is smaller, helping to make it seem farther away. You might also make the whole valley floor slightly darker in color/contrast than the plateau.