"He set about surrounding with a wall of stone those parts of the city which he had not already fortified, which work had been interrupted at the beginning by a war with the Sabines" (Livy, i. 38). "He [Servius] surrounded the city with a rampart, trenches, and a wall, and thus extended the Pomœrium," 578 B.C. (Livy, i. 44). "As the Esquiline and Viminal Hills were both of easy access from without, a deep trench was dug outside them, and the earth thrown up on the inside, thus forming a terrace of six stadia in length along the inner side of the trench. This terrace Servius faced with a wall, flanked with towers, extending from the Colline to the Esquiline gate. Midway along the terrace is a third gate, named after the Viminal Hill" (Strabo, v. 3). "Tullius had surrounded the seven hills with one wall" (Dionysius, iv. 14). The seven hills were not surrounded, strictly speaking. Each hill formed a bastion, and aggers, or curtains of earth faced with stone, were built across the valleys, uniting these bastions. The Esquiline, Viminal, and Quirinal, being ridges jutting out of the table-land and not isolated hills, had one long agger built across their necks. "Some parts of these walls, standing on hills, and being fortified by nature itself with steep rocks, required but few men to defend them, and others were defended by the Tiber.... The weakest part of the city is from the gate called Esquilina to that named Collina, which interval is rendered strong by art; for there is a ditch sunk before it, one hundred feet in breadth where it is narrowest, and thirty in depth. On the edge of this ditch stands a wall, supported on the inside with so high and broad a rampart that it can neither be shaken by battering-rams nor thrown down by undermining the foundations. This rampart is about seven stadia in length and fifty feet in breadth" (Dionysius, ix. 68). This grand agger can be traced almost in its entire extent, as also the smaller aggers. There seems to have been no wall--that is, stone or earth fortification--between the Aventine and Capitoline, the Tiber being considered a sufficient defence. "The city, having no walls in that part next the river, was very near being taken by storm" (Dionysius, v. 23) when Lars Porsena advanced to attack the city, after having taken the Janiculum, intending to cross the river by the only bridge, which, as we know, was defended by Horatius Cocles, and broken down by the Romans in his rear. The walls of Servius Tullius were strengthened at the time of the war with Gabii. "Tarquinius Superbus was particularly active in taking these precautions, and employed a great number of workmen in strengthening those parts of the city walls that lay next to the town of Gabii, by widening the ditch, raising the walls, and increasing the number of the towers" (Dionysius, iv. 54).