When I really want to emphasize height, I create a simplified version of my intended structure in 3D with shadows and render the image to show me how the shadows are cast. Then line up the rendered image with my mapped version of the structure and duplicate the shadows as transparent dark shapes. On my second challenge map in October 2007, I was creating a temple with onion style turrets on the towers like on Russian Orthodox or Muslim temples. I didn't know what an onion turret might look like as a shadow. I did this exact technique to figure that out - it worked.
Since you're a free software kind of gal, Sketchup would be the free 3D app for you - note 3D isn't so easy to learn, though.