Dr. Koldewey suggests that, as the estimate of Ctesias approximates to four times the correct measurement, we may suspect that he mistook the figure which applies to the whole circumference for the measure of one side only of the square. But even if we accept that solution, it leaves the still larger figure of Herodotus unexplained. It is preferable to regard all such estimates of size, not as based on accurate measurements, but merely as representing an impression of grandeur produced on the mind of their recorder, whether by a visit to the city itself, or by reports of its magnificence at second-hand. The excavators have not as yet devoted much attention to the city-wall, and, until more extensive digging has been carried out, it will not be possible to form a very detailed idea of the system of fortification. But enough has already been done to prove that the outer wall was a very massive structure, and consisted of two separate walls with the intermediate space filled in with rubble. The outer wall, or face, which bore the brunt of any attack and rose high above the moat encircling the city, was of burnt brick set in bitumen. It measured more than seven metres in thickness, and below ground-level was further protected from the waters of the moat by an additional wall, more than three metres in thickness, and, like it, constructed of burnt brick with bitumen as mortar. Behind the outer wall, at a distance of some twelve metres from it, was a second wall of nearly the same thickness. This faced inward towards the city, and so was constructed of crude or unburnt brick, as it would not be liable to direct assault by a besieger; and the mortar employed was clay.[23] The crude-brick wall cannot be dated accurately, but it is certainly older than the reign of Nebuchadnezzar, and in his father's time it probably formed the outer city's sole protection.[24] The burnt-brick wall and the moat-lining in front of it date, in their present form, from the age of Nebuchadnezzar, for they are built of his square bricks, impressed with his usual stamp, which are so common over the whole site of Babylon. [Illustration: FIG. 4. GROUND-PLAN OF PART OF THE OUTER CITY-WALL. A: Outer moat-lining of burnt-brick. B: Moat. C: Inner moat-lining of burnt-brick. D: Outer wall of burnt-brick. E: Rubble-filling. F: Inner wall of crude brick, with towers built at intervals across it. The figures on the plan give measurements in metres. (After Koldewey and Andrae.)]