Mesopotamian architecture developed between 3500 BC to 539 BC in the fertile region between the Tigris and Euphrates Rivers occupied by Sumerians, Babylonians, Assyrians, and Persians. Sumerians invented writing and built walled cities like Ur using mud bricks and reeds due to lack of stone. Temples and palaces featured rectangular rooms and some had oval platforms. Later Assyrian architecture emphasized fortified royal palaces and religious buildings with carved reliefs and barrel vaults. Babylon was known for the Hanging Gardens and Ishtar Gate decorated with colored animal symbols.