The War of Emperium is a game feature that allows guilds to fight in order to conquer a Castle to be the headquarters for the guild, and get benefits and special advantages.
Owning a Castle gives your guild several benefits. First off, there is a special Guild Dungeon accessible to everyone in the guild. These dungeons have several strong monsters that in some cases do not spawn in the normal world. Secondly, every night, 4 or more (depending on the economy of the castle) treasure boxes will appear in the castle giving useful items and equipment.
Transcendent (Rebirth) classes cannot enter Non-Trans castles.