The weekly guild war for the Castle in the Valley of Loren. When the battle calls, participants are moved to the siege battleground automatically - there is no server to pick.
Only ancient set / pieces are allowed in the siege. Helm, armour, pants, gloves and boots must all be ancient items - anything else gets you a warning, then removed from the battleground. Weapons, wings, pets, pendants and rings are unrestricted. Ancient Sets come from the White Wizard invasion - see Events.