Ziggurats were important milestones in the evolution of cities. They were massive stone structures built in the  Mesopotamian valley and western Iranian plateau, in the form of a terraced  pyramid-like structure of successively receding stories or levels.In ancient times, they worked as nuclei for urban growth and towns were often established around them.

Important ziggurats are:

•  Great Ziggurat of Ur near Nasiriyah, Iraq
  
•  Ziggurat of Aqar Quf near Baghdad, Iraq
  
• Now destroyed Etemenanki in Babylon (possibly the inspiration behind the biblical story of the Tower of Babel)
  
• Chogha Zanbil in Khūzestān, Iran;
  
•  Sialk near Kashan, Iran.
  

Ziggurats were built by many civilizations namely Sumerians, Babylonians,  Elamites,  Akkadians, and Assyrians . Each ziggurat was part of the temple complex which included other buildings. The precursors of the ziggurat were raised platforms that date from the Ubaid period during the fourth millennium BC.. The latest of Mesopotamian ziggurats date from the 6th century BC. Built in receding tiers upon a rectangular, oval, or square platform, the ziggurat was a pyramidal structure with a flat top. Sun-baked bricks made up the core of the ziggurat with facings of fired bricks on the outside. The facings were often glazed in different colors and may have had astrological significance. Kings sometimes had their own names engraved on the glazed bricks. The number of tiers ranged from two to seven. It is assumed that they had shrines at the top, but there is no archaeological evidence for this and the only textual evidence is from Herodotus. Access to the shrine would have been by a series of ramps on one side of the ziggurat or a spiral ramp from base to summit. The Mesopotamian ziggurats were not places for public worship or ceremonies. They were believed to be dwelling places for the gods, and each city had its patron god. Only priests were permitted on the ziggurat or in the rooms at its base, and it was their responsibility to care for the gods and attend to their needs. The priests were very powerful members of Sumerian society.

One of the best known and best-preserved ziggurats is Chogha Zanbil in western Iran. The Sialk ziggurat, in Kashan, Iran, is the oldest known ziggurat, dating to the early 3rd millennium BC. Ziggurat designs ranged from very simple bases upon which a temple sat, to marvels of mathematics and construction which spanned several terraced stories and were topped with a temple.

According to Herodotus, at the top of each ziggurat was a shrine, although none of these shrines have survived. One practical function of the ziggurats was a high place on which the priests could escape rising water that annually inundated lowlands and occasionally flooded for hundreds of miles, for example, the 1967 flood. Another practical function of the ziggurat was for security. Since the shrine was accessible only by way of three stairways, a small number of guards could prevent non-priests from spying on the rituals at the shrine on top of the ziggurat, such as cooking of sacrificial food and burning of carcasses of sacrificial animals. Each ziggurat was part of the temple complex that included a courtyard, storage rooms, bathrooms, and living quarters, around which a city was built.

References:

http://maps.thefullwiki.org/Ziggurat_truncated

http://en.wikipedia.org/wiki/Ziggurat_truncated

http://quizlet.com/2295142/history-of-architecture-vocabulary-flash-cards/_truncated