The excavation made to obtain the earth for the mount was filled with water from a small rivulet, forming a lake from which the cattle drank, its overflow being carried by an aqueduct along the foot of the Green Mount to fill another great and very deep excavation, made in the same manner as the former. This was used as a fish-pond, containing fish in large variety and number, sufficient to keep the table of the emperor constantly supplied. Iron or copper gratings at the entrance and exit prevented the escape of the fish along the stream. The pond was also stocked with swans and other aquatic birds, and a bridge across its width led from one palace to the other. Such was the palace. The city was correspondingly great and prosperous, and had an immense trade. A thousand pack-horses and carriages laden with raw silk daily entered its gates, and within its workshops a vast quantity of silk and gold tissues was produced. As Hoangti made himself famous by the Great Wall, so Kublai won fame by the far more useful work of the Great Canal, which was largely due to his fostering care, and has ever since been of inestimable value to China, while the Wall never kept out a Tartar who strongly desired to get over its threatening but useless height. Having said so much about the conditions of palace and capital, it may be of interest to extract from Polo's narrative some account of the method pursued in war during Kublai's reign. The Venetian attended a campaign made by the emperor against one of his kinsmen named Nayan, who had under him so many cities and provinces that he was able to bring into the field an army of four hundred thousand horse. His desire for sovereignty led him to throw off his allegiance, the more so as another rebel against the Grand Khan promised to aid him with a hundred thousand horsemen. News of this movement soon reached Kublai, and he at once ordered the collection of all the troops within ten days' march of Kambalu, amounting in all to four hundred and sixty thousand men. By forced marches these were brought to Nayan's territory in twenty-five days, reaching there before the rebel prince had any warning of their approach. Kublai, having given his army two days' rest, and consulted his astrologers, who promised him victory, marched his army up the hill which had concealed them from the enemy, the great array being suddenly displayed to the astonished eyes of Nayan and his men.