highest importance to the spatial interior is attested by the sporadic survival of such buildings as the Praetorium at Musmiyyeh and a room in the Golden House of Nero,[119] where the four-sided and the round dome were placed respectively on piers and on columns; but the final mastery was reserved for early Christian builders of the Hellenistic coast-lands, or developed in the same age in Rome out of methods which were specifically Roman, such as the intersecting barrel vault and construction in concrete. In Rome also the original impulse may have come from the East.[120] 5. In three of the upper rooms in the palace (Nos. 13, 15, and 16, _Hatra_, ii, Figs. 227 and 228, and Plate 10) the roof is formed by means of transverse arches (respectively five, three, and one in number) carrying stone slabs which cover the space between them. This type of roof was universally employed in Syria from Nabataean times until the Mohammadan invasion.[121] It was a simple and a satisfactory method of roofing in stone in a country where centering beams, sufficiently massive to sustain a stone vault, were difficult to obtain. I know no other Mesopotamian example of it in stone, but it was copied in Sasanian brickwork, where the stone slab was replaced by a brick vault running at right angles to the main axis.[122] In this form it finds a place at Ukhaiḍir in room 32, and it continued to be used by Mohammadan builders in the Middle Ages, the most renowned example being that of Khân Orthma, at Baghdâd.[123]