First of all, fortifications are necessarily an abstraction. To be truly accurate, fortifications in any region type besides a city or stronghold would have to be around a bunch of smaller enclaves rather than one monolithic wall around a single place.
Second of all, what regions you can and can't build fortifications in is very heavily influenced by game balance, as opposed to historical accuracy. It used to be possible to build palisades in any region, but this caused cavalry to become effectively useless.
Third of all, in real life you'd also be able to straight-up build a city in a region that's now a rural, and that is not, and will never be possible in BattleMaster. Similarly, some kinds of fortifications will not ever be supported in some places.
All that said, I am interested in ways to make fortifications more interesting than they are today—both harder to walk over, and supporting more complex interactions. I have some changes on the drawing board to how food is handled that should make sieges a genuine possibility, and once that's true, it opens up new options for making city assaults harder.
