A: Sargon's quay-wall. B: Older moat-wall. C: Later moat-wall of Nebuchadnezzar. D: Intermediate wall. E: South fortification-wall of crude brick, probably Imgur-Bêl. F: North fortification-wall of crude brick, probably Nimitti-Bêl. G: North wall of the Southern Citadel. I: Ruins of building, possibly the quarters of the Captain of the Wall. J: Palace of Nabopolassar. K: West Extension of the Southern Citadel. L: Connecting wall. M: Later wall across channel with grid for water. N: Water, originally the left side of the Euphrates. P: Later fortification of Nebuchadnezzar in former bed of the Euphrates. 1-3: Nabopolassar's quay-walls. N.B. The quays and moat-walls are distinguished by dotting. (After Koldewey.)] The extensive alterations which took place in the old citadel's fortifications, especially during Nebuchadnezzar's long reign of forty-three years, led to the continual dismantling of earlier structures and the enlargement of the area enclosed upon the north and west. This is particularly apparent in its north-west corner. Here, at a considerable depth below the later fortification-walls, were found the remains of four earlier walls,[42] the discovery of which has thrown considerable light on the topography of this portion of Babylon. All four are ancient quay-walls, their northern and western faces sloping sharply inwards as they rise. Each represents a fresh rebuilding of the quay, as it was gradually extended to the north and west. Fortunately, stamped and inscribed bricks were employed in considerable quantities in their construction, so that it is possible to date the periods of rebuilding accurately.