The mound of Babil represents Nebuchadnezzar's latest addition to the city's system of fortification, and its construction in advance of the old line of the outer walls was dictated by the desire, of which we find increasing evidence throughout his reign, to strengthen the capital against attack from the north. The mound has not yet been systematically excavated, but enough has been done to prove that, like the great citadel upon the Ḳaṣr, it protected a royal palace consisting of a large number of chambers and galleries grouped around open courts. From this fact it is clear that a Babylonian citadel was not simply a fortress to be used by the garrison for the defence of the city as a whole: it was also a royal residence, into which the monarch and his court could shut themselves for safety should the outer wall of the city itself be penetrated. Even in times of peace the king dwelt there, and the royal stores and treasury, as well as the national armoury and arsenal, were housed in its innumerable magazines. In the case of the Southern Citadel of Babylon, on which excavations have now been continuously carried out for sixteen years, we shall see that it formed a veritable township in itself. It was a city within a city, a second Babylon in miniature.[30] The Southern or chief Citadel was built on the mound now known as the Ḳaṣr, and within it Nebuchadnezzar erected his principal palace, partly over an earlier building of his father Nabopolassar. The palace and citadel occupy the old city-square or centre of Babylon, which is referred to in the inscriptions as the _irsit Babili,_ "the Bâbil place."[31] Though far smaller in extent than Nebuchadnezzar's citadel, we may conclude that the chief fortress of Babylon always stood upon this site, and the city may well have derived its name Bâb-ilî, "the Gate of the Gods," from the strategic position of its ancient fortress, commanding as it does, the main approach to E-sagila, the famous temple of the city-god.[32] The earliest ruins in Babylon, which date from the age of Hammurabi and the First Dynasty of West-Semitic kings, lie under the mound of Merkes[33] just to the east of E-sagila and the Tower of Babylon, proving that the first capital clustered about the shrine of the city-god. The streets in that quarter suffered but little change, and their main lines remained unaltered down through the Kassite period into Neo-Babylonian and later times.[34] It was natural that even in the earlier period the citadel should have been planted up-stream, to the north of city and temple, since the greatest danger of invasion was always from the north. [Illustration: FIG. 5. CONJECTURAL RESTORATION OF THE SOUTHERN CITADEL.