The Millennium Castle Brunestud where Arcueid had sealed herself away is not her original creation, but was actually created in the past by the most powerful True Ancestor, using marble phantasm. That original lord of Castle Brunestud died long ago, but ever since those True Ancestors capable of forming the castle have been uplifted as royalty.

Tsukihime Dokuhon PlusPeriod - TSUKIHIME Dictionary: Brunestud