Actually many servers I was on in the past did different castles for each WoE day.
So like Payon on Sunday and on Weds Prontera and maybe Thursday Alde and on Saturday Geffen. It was kind of a nice setup to me really and they usually put the WoE's at different times, but those were on more internationally based servers.
You can also make sure guilds can only have like 1 castle max if you think its necessary o.O
And no I'm not saying to implement that but saying it has been xD