In the alluvial plains of the Tigris and Euphrates stone and timber suitable for building were rare or unobtainable except by importation. There was an abundance of clay which compressed in moulds and either dried in the sun or kiln-fired, provided bricks for every kind of structure. Besides massive, towered fortifications, the outstanding constructions were temple-complexes or palaces, temples being typical of Babylonian architecture and palaces of Assyrian. Buildings were raised on mud-brick platforms and the chief temples has sacred 'Ziggurats', artificial mountains made of tiered, rectangular stages which rose in number from one to seven in the course of Mesopotamian history.

Apart from the fortifications and ziggurats, buildings of all types were arranged round large and small courts, the rooms narrow and thick-walled, carrying brick barrel vaults and sometimes domes. The roofs were usually flat outside except where dome protruded. Alternatively in early or common place buildings, palm logs supported rushes and packed clay served for coverings or for the best work, cedar and other fine timber was laboriously imported. Burnt brick was used sparingly for facings or where special stress was expected. Walls were whitewashed or, as with the developed ziggurat, painted in colour.

Essentially, architecture was arcuated, the true art with radiating voussoirs having being known by the third millennium BC. For want of stone, columns were not used except in a few instances in late Assyrian Neo-Babylonian work. Towers or flat buttress strips were commonly vertically paneled and finished in stepped battlements above and stone plinths below, with colossal winged bulls guarding the chief portals; in places the alabaster plinths or dadoes of state courts and chambers bore low-relief carving, the walls above them being painted internally with bands of continuous friezes on the thin plaster coverings. Facing with polychrome glazed bricks, introduced by the Assyrians, was another mode of decoration, especially favored by the Neo-Babylonians in lieu of sculptured stone slabs since stone in Babylonia was scarcer than in Assyria.

The architecture in Persians was columnar and thus vastly different from the massive arcuated architecture of Mesopotamian peoples they conquered. Flat timber roofs rather than vaults served for coverings, which allowed columns to be slender and graceful, while with their help, rooms could be large where necessary and of square proportions rather than elongated as the Mesopotamian brick vaults demanded. For ceilings, wooden brackets and beams carried by the columns supported a covering of clay on a bedding of reeds on logs or planks.

The Persians were at first relatively inexperienced craftsmen and drew upon superior skills of the peoples of the empire; many of the usages and features demonstrate derivation from Egyptian, Mesopotamian, Syrian, Ionian, Greek and other sources.

It would be accurate to claim that the architectural character of the major buildings erected during many centuries in Mesopotamia and during the Achaemenian period in Iran, exemplify the two main traditions of the middle east as a whole highland zone respectively. These were the traditions of clay and wood.

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