I think, need add some "free lands" to the game, where players in clan/guild can be make a Castles. And, this free lands need to populate the various chieftains with they clans. So if clan/guild want to make castle, they need to defeat enemy AI clan. Also need add profession like Builder at first they can be make wooden building/walls. After some profession growth they will be able to build a stone building and so on.
Also need other profession, Sieger or Engineer, this people able to build assault machines ( catapults, trebuchets, battering rams). Also builders will be able to fortify castles with some features
So what are the privileges of the clans in their castles?
-Traders
-Profession trainers
-Combat traineers.
-They can build homes in their castle, to increase populate of their castle, and collet tax from NPC
- and etc.