Ziggurat is the name commonly given to the vaguely pyramid-shaped, stepped structures (ziqqarratu) built in many temple compounds in Mesopotamian cities from c. 2200 BCE until c. 550 BCE. Unlike Egyptian pyramids, ziggurats were of solid construction (mostly mud brick and bitumen), lacking internal chambers. The upper platforms of ziggurats supported one or more shrines, at least one generally sacred to the patron god of the city in which the ziggurat was located. Where indications have survived, they reveal that access to the upper platform was by either exterior triple stairway or by a spiral ramp. Ziggurats were generally rectangular structures, typically averaging 40 x 50 m at the base. No ziggurat has retained its original height.
The largest preserved ziggurat is that of Al-Untaš-Napiriša (modern Choga Zanbil) in Elam (south-western Iran). Constructed in the 13th century BCE, the ziggurat measures 100 m by 100 m and is preserved to half its original estimated height of 48 m. Access to this ziggurat was made uniquely by means of internal stairways.
The best preserved ziggurat, however, is that of the moon god Nanna at Ur. Measuring some 64 x 46 m at the base, the ziggurat was originally some 12 m in height over three stories (this was later increased to seven stories by the Babylonian ruler Nabonidus in the 6th century BCE).
George, Andrew R.
1993 House Most High: The Temples of Ancient Mesopotamia, Mesopotamian Civilizations 5 Winona Lake: Eisenbrauns, 1993.