The numerous invasions which Babylonia had suffered from the Assyrians must have been held in lively recollection, and the founder of the new kingdom could not omit to bestow his earnest attention on the mode of preventing such dangers for the future. They were only possible on the side of Media. So far as the difference of force in comparison with Media was not removed by the better frontier, the more homogeneous population, and the greater productive power in Babylonia, it was necessary to attempt to remove it by the erection of fortresses in the land. As the attacks of the Assyrians had taken place from the North, the attacks of the Medes were also to be expected from that quarter. Mesopotamia might, in case of necessity, be abandoned, if the native land were made secure. Babylonia had excellent bulwarks on the East and West in the Euphrates and the Tigris; in the North the line of canals, especially the new and broad canal, Nahr Malka, formed a similar protection. The basin of Sippara was not merely constructed with a view to the cultivation of the soil and the navigation; it was at the same time calculated that the supply of water contained in it was sufficient to change the most northern of these canals into deep watercourses. The sluices were guarded by the fortress of Sippara.[692] How destructive this basin would one day be to his metropolis, how it would render vain the fruit of all his labours, Nebuchadnezzar never dreamed. If every hostile power in the East and West had to cross a wide river in the face of the Babylonian army, the two rivers from Sippara downwards could now be filled by opening the great reservoir and by closing the sluices of the Pallakopas in such a manner that it became more difficult than ever to cross them. The same was the case with the canals. But the difficulties here were not so great, and they did not satisfy Nebuchadnezzar. In order to strengthen the defence of the northern border, in order to protect the basin of Sippara, on which depended the filling of the upper canals and the feeding of the lower course of both streams, to make more secure the fertile part of Babylonia, Nebuchadnezzar built a strong wall, extending from the Euphrates to the Tigris, above the four canals and the fortified Sippara. This fortification the Greeks call "the Median wall." It was, in fact, intended to meet the attacks of the Medes. Had Nebuchadnezzar chosen for the line of the wall the point at which the two rivers most nearly approach each other, the length of it would have been little more than 25 miles; but as Sippara and all the land of the canals had to be protected, the wall must have been placed farther to the north. It appears to have left the Euphrates at Ssifeira below the modern Feludsha, and, extending in a north-easterly direction, it reached the Tigris at some distance above the modern Bagdad. The length was thus from 60 to 75 miles. The wall was constructed of burnt bricks joined