Ziggurats are, architecturally, the Mesopotamian equivalent of the Egyptian pyramids: large artificial square mountains of stone. They are equally ancient. But there are two differences: a ziggurat was not a tomb but a temple, and ziggurats were built well into the Seleucid age, whereas the building of pyramids came to an end after c.1640 BCE. Ziggurats are, briefly, temple towers.
Our word ziggurat is derived from ziqqurratu, which can be translated as "rising building" (Akkadian zaqâru, "to rise high"). Some of them rose very high indeed. The temple tower known as Etemenanki (the 'House of the foundation of heaven on earth') in Babylon was 92 meters high. Even larger was the shrine of the god Anu at Uruk, built in the third or second century BCE. The best preserved temple tower is at Choga Zanbil in Elam, modern Khuzestan in Iran.
Ziggurats played a role in the cults of many cities in ancient Mesopotamia. Archaeologists have discovered nineteen of these buildings in sixteen cities; the existence of another ten is known from literary sources.
They were always built by kings. In third millennium BCE Mesopotamia, there was a conflict between the two great organizations, the temple and the palace. By building ziggurats, the king showed that he could perform more impressive religious deeds than the priesthood.