Before arriving at the Cœlian Hill, it had to be conveyed to and along the Cœliolum (now the Lateran Hill). It had come through the gardens of the Sessorium, as we have seen, in the two separate _specus_, after it had entered Rome at the extreme eastern corner, on the north side of these gardens, which are nearly half a mile long. These are _the palace gardens_ mentioned by Frontinus. The Sessorium with its gardens (now the monastery of S. Croce in Gerusalemme) had been one of the two Prætorian camps, this one being at the south end of the great _agger_ of the Tarquins, which formed the outer _mœnia_ of Rome on the eastern side. The one, called the Prætorian Camp, is at the north end of the same great bank. Each of these fortified camps was surrounded by a great wide and deep foss or trench. These great banks and trenches are usually mistaken for natural hills and valleys; but natural hills and valleys are not merely high banks fifty feet high and perhaps a hundred feet wide, nor are natural valleys long narrow trenches of the same dimensions, running on each side of the great banks, being in fact the trenches from which the earth has been thrown to form the banks. Such a bank with such trenches can be clearly traced all round Rome by eyes that are accustomed to examine such things, although modern buildings have disguised them so much that an ordinary observer does not see them. People cannot see over modern walls of twelve feet high, still less over the great Wall of Aurelian fifty feet high, and they do not think of comparing the level of the ground on each side of that wall, nor can they easily do so. These great ancient earthworks were extremely convenient for the engineers of the Aqueducts, which were brought upon the high banks at the farthest corner. They were carried along the bank on the north side of the Sessorium, from its north-east corner to the south-west angle, where the great bank of the Tarquins joins on to it. Some branches of the Aqueducts, perhaps the main stream in some cases, were then carried along this high bank of the Tarquins to the north, as far as the other Prætorian Camp, and beyond it along the outer wall of Rome, as we have seen in the case of the Anio Vetus, the Marcia, Tepula, and Julia. These did not pass through the Sessorium, but parallel to it about a hundred yards on the north of it. The Claudian arcade had been at about that distance on the north of it, all the way from the Piscinæ into Rome. The Marcian arcade was made nearly over the Anio Vetus, which ran between that arcade and the Claudian, but much nearer to the Marcian.