Mesopotamia, known as the "land between the rivers", was the site of some of the earliest urban civilizations beginning in the 6th millennium BC. The Sumerians developed one of the earliest writing systems, cuneiform script, using wedge-shaped symbols pressed into clay tablets. Architecture featured ziggurats, large temple towers built in receding stories, and the society was organized with the king and priests at the top and slaves at the bottom. Hammurabi further developed Mesopotamian civilization in the 18th century BC by creating one of the earliest legal codes.