Ronda, Spain has some cool bridges across a gorge c. 100 meters (c. 328 feet) deep between parts of the town. It first came to my attention as a screensaver on my Win 10 PC, so I had to look it up.
Playing with the Google map Earth view and the 3-D features gives a very cool view of the “New Bridge”, Puente Nuevo.
I have in mind a walled city atop the cliffs, split by the gorge, with one part surrounded by steep cliffs, and the bridge across the gorge is from an area that slopes away, providing the only ground access. Massive gates with a drawbridge to prevent direct access to the city would deter all but the most determined and well-equipped enemy.
The citadel area would not have room to spread out, but could grow up, and have sewers and passages carved out of the stone beneath. Over time, these clandestine ways in allow thieves and smugglers their own way in and out of the city. Over time, the knowledge of these ways may be lost to officials and laxity in patrols could lead to enemies or creatures getting in unobserved.
The other half of the city across the bridge would have room to grow beyond the walls, limited by the cliffs and the gorge. There would be fewer tall building and less digging below, since it is easier to just spread out here. Tall buildings and excavations below would only appeal to those that have nowhere else to spread out. However, the city government, local noble, etc. would prevent buildings being too tall to prevent them being an archery platform in the event of an invasion or siege.
I wish I had time to dig into developing this right now, but I’m gathering notes for the four games I’ll be running a UCon in a few weeks. I have to get it done now since I will be at Gamehole Con the weekend before UCon. So I also have to gather the stuff for the games I am signed up for at Gamehole.