The caravanserai type, when once it had established itself on the Arabian limes, was not to be ousted, but its later application is not only to fortress and barrack, but to genuine lodgings for caravans. In the Roman or Byzantine caravanserai of Khân al-Zebîb enough remains to show that the interior buildings were placed round the encompassing wall.[208] At Umm al-Walîd this interior arrangement is clearly preserved;[209] at Umm al-Rasâs baits, not unlike those of Qasṭal, are linked to the wall,[210] and the plan of a later building at Khân al-Zebîb (it is probably Moslem) differs not at all from that of a small modern caravanserai.[211] Khirbet al-Baiḍâ (see above, p. 56) belongs to the same group, but from its geographical position it must be regarded as a military station rather than as a true caravanserai, though it may have served both purposes. To what cause is the singularly rapid change from Roman camp to Asiatic caravanserai to be attributed? The answer is obvious. On the Arabian limes the builders were brought into contact with a strong Asiatic tradition; they were probably themselves local workmen, and they orientalized the Roman scheme. They applied from the first their own system of flanking towers to the defences; they grafted an injunctive plan on to the Roman camp plan, and they ended by discarding the latter in favour of the former. The covering of dead ground by means of flanking towers and crémaillères goes back in western Asia to the earliest times. The plan of the acropolis of Gudea, drawn upon a tablet which is placed in the lap of a statue of the patesi of Lagash, exhibits, in the middle of the third millennium B.C., a system of fortification so fully developed that scarcely a dead angle exists in the whole circuit of the walls (Fig. 27). In the science of military engineering even Egypt would seem to have lagged behind Chaldaea, for the advantage of flanking towers was not understood there until the Asiatic expeditions of the Eighteenth Dynasty had taught the Pharaohs how to correct the defects in the unbroken lines of their massive defences.[212] In the Assyrian reliefs, double and triple rings of walls set thick with towers surround the towns; towered walls are represented in the ground-plans,[213] and excavation has proved the existence of rectangular towers in the walls of Khorsâbâd and of Assur.[214] A chemin de ronde, loopholes, and machicolations have been found _in situ_ in the walls of Assur, together with traces of crenellation,[215] and all these features, as well as hourds projecting from the battlements, and the ladders and battering-rams which they were intended to counteract, are familiar upon Assyrian reliefs. Rounded towers have not been revealed by Babylonian or Assyrian excavations. They belonged to a later age or perhaps to a different sphere of culture, the Hittite or Syrian. But Dieulafoy observed them on the Achaemenid fortifications of Susa;[216] and at