A great renaissance in architecture took place under the Sassanid dynasty, which ruled Persia from 224 until 651. Construction was radically different from that of the Achaemenian period. Walls were built of burnt brick or small stones bound with mortar; barrel vaults of brick were used to span rooms and corridors; and domes were erected over the large halls. The principal features of the plan of the palaces at Persepolis were adopted, but the various rooms were enclosed within a single building.
Thus, the same building incorporated a public audience hall, a smaller private audience hall, and a complex of lesser rooms. Remains of the major monuments of Sassanian architecture include the ruins of domed palaces at Firuzabad, Girra, and Sarvestan, and the vast vaulted hall at Ctesiphon. The large site of Bishapur was systematically excavated in the mid-20th century by the Archaeological Service of Iran. Palace sites have also been excavated at Qais, Hira, and Damghan. Other ruins include bridges at Dizful and Shushtar and a number of small temples built at various locales for the Zoroastrian worship of fire.