A region has allegiance. Initially this is to noone.
A regions allegiance can be changed by some method so that it now pays taxes to
a castle/citadel. [Possibly as long as there is a route through alligned regions
from this region to the one with the castle in it.]
So essentially, you can build arbitrarily large kingdoms, as long as they are
centred around a citadel or castle.
Now I think that the tax revenue gained from a region should be highest for the
region being protected by the building itself, and less for any other region.
Changing the allegiance of a region could require one or more of:
a) A show of military might
b) Oratory
c) Magical (priestly) ritual
...to gain allegiance
a) Consistant pillaging for 6 months
b) Oratory
c) Magical (priestly) ritual
d) A show of military might
e) A sufficiently long time
...to lose allegiance.
Of course regions could be ceeded from one controller to another.
Also castles themselves could swear allegiance to citadels, forwarding a
percentage of their tax income (50% sounds reasonable) to the citadel.
A related, possibly alternative, suggestion:
Troops can be left in buildings without a leader, and will collect taxes for the
owner. Perhaps there is a nominal Captain (without any skills) who takes the
place of the noble.
The advantage of this is that a single faction can control more castles than
they have nobles. There would probably be a morale hit (they would surrender at
40% losses instead of 50% for instance).
Just another two for the dartboard.
John.
Faction Curumo the captured.