Small update since last time.
I'm happy with how my houses are coming along and need to do some city planning before I make to much progress. I needed to do a quick mockup and make sure I'll be able to create enough city walls to surround a portion.
Here is the beginning of that process...
Extruded Polystyrene sheet (2 inch thick) cut down so that the width is the length of my wall sections (11"). I decided on about a 7 inch wall height but that might change as I test them next to my buildings.

Here they are chopped down into the sections and ready to be sanded.

After that I scribed rock texture into them with a ball point to represent the rock base Greek walls were built on. This is pretty dull work and was accomplished while watching Alien Directors cut. Apparently in 5th century Greece they put about an 8' base of heavy rocks as a foundation and then built the mud brick wall above that (plastered over). You can see the height of the wall next to the figure. Currently it is to scale but sometimes you have to fudge buildings smaller to make them work in miniature scale.

Next up...more scribing, more planning, more buildings!
@Alxbates - I use basic wall spackle applied with a putty knife and then dry brushed.