In the Seleucid period, Mesopotamia served as a base for the Seleucid kings' attempts to extend their political and commercial power into the Persian Gulf region and along the eastern coastlands of Arabia. The Parthians and then the Sasanians made Ctesiphon, on the eastern bank of the Tigris in central Mesopotamia, their capital and the centre from which Iranian power radiated over Aramaic, and then increasingly Arab, Iraq. The 6th century was a propitious time for Persian intervention in South Arabia. In the sphere of architecture, Persian influence on the buildings of the Lakhmids, such as the palace of Khawarnaq, must have been decisive, and Persian models must have dominated the architecture of early Islamic Iraq. Persian artistic influences also penetrated across the Syrian desert to the structures of the Umayyad caliphs on the fringes of modern Syria and Jordan, where there was a symbiosis with the local hellenistic and Byzantine artistic and architectural traditions.