A rectangular stepped temple tower in which each story is smaller than the one below, leaving a terrace around each floor - characteristic of Sumerian, Assyrian, Elamite, and Babylonian architecture. It is thought to symbolize a mountain; the tower was the high place of the god, the link between earth and heaven. Its structure was of mudbrick with a burnt brick casing, with stairways giving access to a small shrine at its summit. The arrangement of the stairways leading from one stage to another varied greatly from one monument to another. The best known examples are those of Ur, Babylon (Tower of Babel), Eridu, Aqar Quf, Borsippa, Dur Kurigalzu, Uruk, and Choga Zambil (Zanbil) in Elam. It originated with the Sumerians and the first ziggurats were modest constructions but, by 2000 BC, more imposing examples dominated the great cities of southern Mesopotamia.