The Agger of Servius, which has now been so much levelled and destroyed, ran between the Church of S. Maria degli Angeli and the present walls. This enormous rampart has been described by Dionysius. He says that the ditch outside was more than one hundred feet broad at the narrowest part and thirty feet deep, and that a wall stood upon the edge of the ditch supported by the agger, which was of such massive strength that it could not be shaken down by battering rams or breached by undermining the foundation. Dionysius gives the length of the agger as seven stadia, and Strabo as six, which, taking the stadium at 202 yards, nearly corresponds to 1400 yards. The breadth he states at fifty feet. That this ditch and wall were the work of some of the later kings there can be no doubt, but it cannot be determined what part each took in their erection. The final completion of the whole undertaking is ascribed to Tarquinius Superbus, who deepened the ditch, raised the wall, and added new towers. The additions made by him can be distinguished in the portion brought to light by the modern excavations in the railway cutting.[109] Excavations which have been made in this part of the agger from time to time, and the extensions of the city in this direction have brought to light an enormous wall buried in the earth, constructed of huge blocks of peperino. This is probably the wall mentioned by Dionysius, which in his time stood outside the rampart on the edge of the ditch. The remains of buildings of the imperial times have been found placed upon and outside of this wall, and it is probable that the whole ditch is now filled with such remains, and most part of the wall buried in them. The central railway station stands close to this agger, and cuttings have lately been made through it to make room for the station, by which new portions of the wall have been disinterred. All these excavations have proved the truth of Dionysius’s description, the wall having been found on the outer side of the original agger, which is easily distinguishable from the rubbish in which it is buried by being composed of clean soil unmixed with potsherds and brickbats. The wall probably ran from the southern end of the agger along the back of the Esquiline and Cælian in the direction of the modern Via Merulana and Via Ferratella. In this portion must be placed the Porta Querquetulana and the Porta Cælimontana, but their exact situation is unknown.[110] [Sidenote: The Prætorian Camp.]