After all that has been observed above, the attack of Cyrus could not be unexpected by Nabonetus, and we also see from Herodotus that it had been long foreseen, and provisions for many years had been collected in Babylon--according to Xenophon there was sufficient for twenty years. We find, moreover, that the fortifications of the city had been completed; the great extent which Nebuchadnezzar had allowed for the wall of the city must have enclosed a wide breadth of country, or at any rate pastures large enough to maintain numerous herds of cattle. And Nebuchadnezzar had not merely made the metropolis the fortress and strong camp of the kingdom, which could both receive and protect the military forces, he had covered the northern edge of the Babylonian land by a fortification of a hundred feet in height and twenty in thickness, which extended from the Euphrates to the Tigris. Behind this wall were the four great canals which connected the Euphrates and Tigris; and, protected by the great wall, there lay on the Euphrates at Sepharvaim, the reservoirs by which the level of the Euphrates could be raised or lowered, and the canals fed--the basin of which Nebuchadnezzar had availed himself in building his bridge over the Euphrates,--works which Herodotus, we do not know on what authority, but very erroneously ascribes to Nitocris, a queen of Babylon. By this wall, and the canals, which it would be necessary to dam up, any attack on the heart of Babylonia from the direction of Mesopotamia would be rendered almost impossible. The Tigris after leaving the mountains of Armenia, above the ruins of Nineveh, is not difficult to cross in the summer, yet an attack from this side would encounter almost insuperable difficulties, and even if they were overcome the attacking army would be involved in a labyrinth of canals, in which the cavalry of the Persians could be of little use. Hence Babylonia could only be reached by crossing the Tigris and Euphrates below that fortification and the canals,--a difficult task. If Cyrus attempted to cross both rivers above this point, and then march down the western shore till he was below the "Median wall," he would sacrifice altogether his communication with Persia, he would have to march southwards through the Syrian desert, and then force the passage of the Euphrates, in the neighbourhood of the metropolis, _i.e._ in the face of the enemy's power, while he at the same time would find himself in the midst of an extensive system of canals, and of the swamps which lie along the Euphrates between Babylon and the sea (I. 300, III. 359).