Also not a city-scape but, I recently completed a back corner with a tunnel through a hill. I tried a mirror first but since it would be forward facing I decided against it. I found an image online that was close to the look of the road I was building and cut it to size and glued it to the back of the portal. The image is recessed about 2" to give some depth.
This is the print, the arch and columns are not visible on the finished product.
Bob
