When Nebuchadnezzar rested after his conquests, secure in the subjugation of his rivals, and in the possession of his vast kingdom, he gave himself up to the material improvement of Babylon and the surrounding country. The city as he left it, at the end of his reign of forty-three years, was built on both sides of the Euphrates, and covered a space of four hundred square miles, equal to five times the size of London. It was surrounded by a triple wall of brick; the innermost, over three hundred feet high, and eighty-five feet broad at the top, with room for four chariots to drive abreast. The walls were pierced by one hundred gate-ways framed in brass and with brazen gates, and at the points where the Euphrates entered and left the city the walls also turned and followed the course of the river, thus dividing the city into two fortified parts. These two districts were connected by a bridge of stone piers, guarded by portcullises, and ferries also plied between the quays that lined the river-banks, to which access was given by gates in the walls. Nebuchadnezzar's palace was a splendid structure covering a large space at one end of the bridge. In the central court were the Hanging Gardens, the chief glory of the city, and reckoned one of the wonders of the world. No clear idea can be formed of these gardens from any description that has come down to us, but it would appear that arches eighty feet high supported terraces of earth planted with all the skill for which the gardeners of the East were famous. We are told that they were built for the pleasure of Queen Amuhia, who, as a Median princess, missed her native mountains, but a more commonplace explanation is that they were carried so high to escape the mosquitoes that swarmed on the lower level. Various splendid edifices, chiefly religious, adorned the great squares of the city: the temple of the god Bel, enriched by the spoils of Tyre and Jerusalem, was the especial pride of Nebuchadnezzar. It rose in a succession of eight lofty stages, and supported on the top a golden statue of the god, forty feet high. Still another temple of Bel was built in seven stages, each faced with enamelled brick of one of the planetary colors; the topmost one of blue, the color dedicated to Mercury or Nebo, the patron god of Nabopolassar. But the most important of the civic undertakings of Nebuchadnezzar was the extension of the great system of canalization by which the barren wastes of the Babylonian plain were made to rival the valley of the Nile in fertility, and become the granary of the East. The whole territory was covered with a network of canals fed by the Tigris and Euphrates, and used for both irrigation and navigation. One branch had already connected Nineveh with Babylon, and another constructed by Nebuchadnezzar united Babylon to the Persian Gulf, running a distance of four hundred miles. This is still to be traced in a portion of its length.