In another forum I've asked - but this is the best place to file this query I think:

I'm wondering how exactly the banners on the fortifications can be changed. Stone walls, the banners over those gates, for example.

Also asked here:
https://forums.totalwar.org/vb/showthread.php?t=51613

I would think they would be the second set of banner textures (main set is in models/textures) that you can find in models_buildings/textures, but I can't get them to change yet. I've seen somewhere that they were hard to change - does anyone have any information on this at all?