"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).
"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).