Very nice terrain and modular. I tend to overthink terrain and then I went and picked up the GW Terrain Book from circa 2008 for $2 at a con and saw some nice, simple solutions like toweling. Unfortunately this lead me to more thinking. I made up a 4 x 4' gaming mat made out of felt with foam applied with spray matte medium and it works great but was a little awkward to negotiate in our d&d game where, after character sheets and drinks are accounted for we don't have 4' across. Am now thinking about smaller pieces of felt with pieces of other materials (here's where towels may come in) to cover the edges where two felt mats meet. Anyway, what you have there looks great and an ideal size for an rpg encounter.
BTW who makes those walls? I foolishly made 5 6" lengths out of real stones which look great but not worth the effort. Am looking around for some.