A castle cost's about 50-100k in gold (or more, but please not less, would want it to be special and not just common like halls). A castle has 5-10? times the size of a hall.
In a GvG you would have to siege the castle. The apposing party gets army soldiers NPC's who help fighting. The walls of the castle have to be breached. You do that by repairing the catapults, and fire those at the walls (automaticly aimed at the walls already) and do that untill the wall's hp are at 0. Then it would be like a normal GvG, enter the castle, and try to kill the lord. Dont really know how the defending role should be tho..I mean with all the defences of the castle itself, defending would be easy

But anyway, this would drasticly change GvG's and i dont know if thats really what we'r looking for...But then again, in my oppinion this would enchance the tactics for GvG's (altho i dont really know about the defending role innit).