Love this idea, so i can show off my strength and get benefits.
But i have a suggestion. How about the regional castle owner also provide benefits to all of those in the region, almost like a head of county. And in return if the region own is liked, the regionals will provide the castle with troops at the garrison to defend it.
Also as a possible counter to the offline this, what if it can be taken over at all times but has a larger percentage of garrison possible?
p.s this sounds as if it came from stronghold kingdoms, but the idea was given to me from a friend.