The fortress is now finished. As you can see from the pictures it's built as a set of modules that can be combined in different ways to provide a variety of forts.
Overall there are two gateways (one large and one small), six wall sections, two small towers and two artillery towers
above & below, the large gateway
An artillery tower clearly showing the ramp and how it fits next to a wall section
I'll probably have to add some more sections as needed for particular games.
The textured finish is a very old pot of artex paint, still going strong after more then 20 years, everytime I open it I expect to find it hard, but it's always still usable. It is white but I gave the whole surface a quick brown/black ink wash to tone it all down.