I've been generally improving forts around Skyrim (adding quartermasters so they're actually useful to you, that sort of thing), with a particular fixation on Fort Sungard in The Reach, because it is possibly the biggest and most strategically valuable of all of them.

I connected the Annex balcony to, what is, in vanilla, just a staircase to nowhere north of the fort's gate (to make the quartermaster more easily accessible, since the forge and sharpening wheel and everything are on that balcony). And of course the place has a great view of Whiterun Hold.

I navmeshed the new section of wall, and finalised it, only to find that NPCs won't walk on my new wall. I couldn't find any problems, and I finalised it again just in case I'd forgot, but no change. Does anybody know how I can make the damn navmesh work?

you ever checked your navmesh (navmesh -> select by triangle -> check), and you ever checked you editorwarnings.txt for navmesh errors? (first go to cell, enter navmesh mode, then open error file for best results)