Lynn,
Thank you Lynn and, yes, I did contemplate using wood for the retaining walls and gave both, the wood and what I have used, a great deal of thought before making the final decision. In fact, that thought process revolved around exactly what you referred to regarding access, and so forth, to the buildings.
What I concluded was it was going to be easier to build access points (steps) into the plaster mold retaining walls than it would be using wood. The other thing I considered, taking into account that I 'pour' my plaster fairly wet (a thickish cream consistency) was how much water absorption the wood would take and; there by, perhaps warping it a little too much. The third thing I took into account was that I have the mold for the 'cobble stone look' and a ton of plaster, so economically it was going to be the better choice to use the plaster mold.
Doing the steps, access points, for the levels will be minimal. There will only be 4 buildings on the ring road, so I will only need 1 central access point along with the access points at either end of those buildings. The closer end (in the photos) will be at ground level, so there will only need to be 2 sets of steps leading up from the road to the footpaths and buildings, one centrally located and one at the farthest end. The same will apply for the main street buildings, which are only elevated on one side.
My intention, once the retaining wall has dried for a couple of days, is to 'carver out' the access points for the steps, which will also be plaster to replicate stone, concrete or what ever I end up with.
While this is going to be a lot of work, some of which is going to be a little fiddly, I think it is going to give the town a more interesting look than having it on a flat, level patch of ground. I also like to make things hard for myself
Once I have all of the retaining wall in place, I am also thinking about giving the ring road a grade, even if only raising it to a highest point of maybe an 1/8 or so of an inch. My thinking is to reduce the 'impact' of the retaining wall a little, especially toward the top end of the photo. That is still on the drawing board though with no absolute decision having been made as yet.