This broad roadway on the summit of the wall, which was of world-renown owing to the descriptions of it given by classical writers, was of the greatest importance for the protection of the great city. It rendered possible the rapid shifting of defensive forces at any time to that part of the wall which was specially pressed by attack. The line of defence was very long; the north-east front, which can still be measured, is 4400 metres long, and on the south-east the ruined wall can be traced without excavation for a length of 2 kilometres. These two flanks of the wall certainly extended as far as the Euphrates as it flowed from north to south. With the Euphrates they enclosed that part of Babylon of which the ruins exist at the present time, but according to Herodotus and others they were supplemented on the other side of the Euphrates by two other walls, so that the town site consisted of a quadrangle through which the Euphrates flowed diagonally. Of the western walls nothing is now to be seen. Whether the traces of a line of wall to the south near the village of Sindjar will prove to have formed part of them has yet to be ascertained. [Illustration: FIG. 2.—Part of the outer city walls; ground-plan. ] The excavations carried on up to the present time have yielded no surrounding walls beyond this fortification. The circuit extended for about 18 kilometres. Instead of this, Herodotus gives about 86 kilometres and Ctesias about 65 kilometres. There must be some error underlying this discrepancy. The 65 kilometres of Ctesias approximate so closely to four times the correct measurement that it may well be suspected that he mistook the figures representing the whole circumference for the measure of one side of the square. We shall later turn more in detail from the testimony of the ancient writers to the evidence of the ruins themselves. Generally speaking, the measurements given are not in accordance with those actually preserved, while the general description, on the contrary, is usually accurate. Herodotus describes the wall of Babylon as built of burnt brick. To an observer from without it would no doubt appear as such, as only the top of the inner mud wall could be seen from outside. The escarp of the fosse was formed of the square bricks that are so extraordinarily numerous in Babylon, that measure 33 centimetres and bear the usual stamp of Nebuchadnezzar. Those of the brick wall are somewhat smaller (32 centimetres) and unstamped. These smaller unstamped bricks are common previous to the time of Nebuchadnezzar, but nevertheless they may very well date from the early years of his reign, as we shall see farther on. To what period the mud-brick wall may be assigned we do not yet know; it is certainly older. It apparently possessed an escarp, of which there are some scanty remains within the great brick wall. It appears to have been cut through on the outside by the latter.