There is a 4th Castle which isnt used.
It can be a weekly Event or once in a month, a Guild can register and have 10 to 20 (15 would be best, a whole party) players attends, make it a Guild vs Guild for the 4th Castle.
People will have to use their own character buffers to win the fight, as party buffs are supposed/were meant to be used.
I can see what I can do with GM and suggest it as "Event"