From ArtAges History of Architecture: To the east of Egypt another civilization appeared about 3000
BC, that of the Sumerians in the river valley of the Tigris and Euphrates called Mesopotamia, or the "land between the rivers." This too was a highly organized culture capable of carrying out large irrigation and construction projects. But it differed from Egypt in two respects: it had no stone with which to build, only river clay, so that its architecture is entirely in brick; and it had no single divine ruler but was divided into a number of independent city-states and worshiped unseen gods.Sumeria. The Sumerian temple was a small brick house that the god was supposed to visit periodically. It was ornamented so as to recall the reed houses built by the earliest Sumerians in the valley. This house, however, was set on a brick platform, which became larger and taller as time progressed until the platform at Ur (built around 2100 BC) was 150 by 200 feet (45 by 60 meters) and 75 feet (23 meters) high. These Mesopotamian temple platforms are called ziggurats, a word derived from the Assyrian ziqquratu, meaning "high." They were symbols in themselves; the ziggurat at Ur was planted with trees to make it represent a mountain. There the god visited Earth, and the priests climbed to its top to worship.
The ziggurat continued as the essential temple form of Mesopotamia during the later Assyrian and Babylonian eras. In these later times it became taller and more towerlike, perhaps with a spiral path leading up to the temple at the top. The Greek historian Herodotus wrote that the main temple of Babylon, the famous Tower of Babel, was such a tower divided into seven diminishing stages, each a different color: white, black, purple, blue, orange, silver, and gold.
Sumerian Art and Architecture [scroll down for Architecture]