The Great Ziggurat of Ur is a massive stepped pyramid built during the Early Bronze Age by King Ur-Nammu in honor of the moon god, Nanna. Unlike Egyptian pyramids, ziggurats were not tombs but rather religious complexes designed to serve as a bridge between the heavens and the earth. The structure consisted of a series of receding tiers, built with a core of mud-brick faced with baked bricks set in bitumen (a natural tar). At the very top stood a shrine, accessible only to priests, where the deity was believed to dwell. The Ziggurat was the administrative and spiritual heart of the city of Ur, one of the most powerful Sumerian city-states. Its sophisticated design included 'weeper holes' to allow moisture to escape the interior, preventing the structure from collapsing. Today, it stands as one of the best-preserved architectural remnants of the Sumerian civilization.