LM11 "Medieval Warfare" from HOB's Long March pack has extensive SSR for city walls.
Although the scenario uses DASL boards, the use of a stretch of road to represent a section of city wall is quite innovative.
https://www.aslscenarioarchive.com/scenario.php?id=60563
At its simplest, a city wall in LM11 is a level one Location with a +2 TEM and an inherent stairwell. You could restrict access to a stairwell to one side of the wall and even particular hexes. However, if units did not fire from the walls, there is no need for stairwells and you're primarily dealing with a level one obstacle.
Beyond that, the SSR could easily get away from you. Unless the historical situation cites it, there's no reason, for example, to allow Scaling (B23.424).
Another place to look for ideas might be V7. Gutted Buildings, especially the effects of being roofless (7.2). For instance, one could treat a series of buildings as Gutted to represent a higher or variable-height wall, with only a top level Location reached via quasi inherent stairwell Locations, not unlike the way a Factory Rooftop Access Point (B23.87) works.
Again, no shortage of ideas. Just some work to distill an SSR down to a few lines of text.