"On the eastern side it is bounded by the Agger of Tarquinius Superbus, a work of surpassing grandeur; for he raised it so high as to be on a level with the walls on the side on which the city lay most exposed to attack from the neighbouring plains. On all the other sides it has been fortified either with lofty walls or steep or precipitous hills; but so it is that its buildings, increasing and extending beyond all bounds, have now united many other cities to it" (Pliny, iii. 9). "After Camillus had driven out the Gauls, both the walls of the city and the streets were rebuilt within a year" (Plutarch, "Cam." 32). "The legions being brought to Rome, the remainder of the year was spent in repairing the walls and the towers," 350 B.C. (Livy, vii. 20). "They received a charge from the senate to strengthen the walls and towers of the city," 217 B.C. (Livy, xxii. 8). After the republic was firmly established, and the boundaries of the state enlarged, the walls of the city became obsolete, and it was to all intents and purposes an open city until the time of Aurelian. "All the inhabited parts around it [the city], which are many and large, are open, and without walls, and very much exposed to the invasion of an enemy. And whoever considers these buildings, and desires to examine the extent of Rome, will necessarily be misled, for want of a certain boundary that might distinguish the spot to which the city extends, and where it ends. So connected are the buildings within the walls to those without, that they appear to a spectator like a city of an immense extent" (Dionysius, iv. 13). FOURTH WALL--THE WALL OF AURELIAN. From the time of Servius to Aurelian the city, though much enlarged, had no new wall, though the boundaries had been extended. To continue our last quotation from Dionysius, who died 7 B.C., this is evident. "But if any one is desirous to measure the circumference of it by the wall--which, though hard to be discovered, by reason of the buildings that surround it in many places, yet preserves in several parts of it some traces of the ancient structure--and to compare it with the circumference of the city of Athens, the circuit of Rome will not appear much greater than that of the other" (Dionysius, iv. 13). The Pomœrium, or city bounds, was enlarged, as we know, by several emperors, some of their _cippi_, or boundary-stones, being still _in situ_; but there was no wall. Where the roads crossed the line of the Pomœrium, gates were built, between which there were no walls. The Romans considered the rivers Tigris, Euphrates, and Danube, the desert and the ocean, as the walls of Rome. "When he [Aurelian] saw that it might happen what had occurred under Gallienus, having obtained the concurrence of the senate, he extended the walls of the city of Rome" (Vopiscus, in "Aur.," 21). "Thus also Rome was surrounded by walls which it _had not before_, and the wall begun by Aurelian was finished by Probus" (Zosimus, i. 49).