They always failed. He had a lot to say on the subject. Here is a very pertinent quote from his book. Particularly ominous is the last paragraph:

“When Constantine marched on Rome in 312, his opponent Maxentius did not rely on the walls but rather marched out to meet his foe. The purpose of the walls was to protect the city until an army could come to its rescue, not to serve as a sufficient defense in their own right. But there is a deeper meaning to these walls. For centuries the majestic grandeur of Rome’s strength had been flaunted in the open roads into the city. It had been six hundred years since the city was last besieged, and for centuries an invasion of Italy had been inconceivable. Every Roman could see this power and this confidence in the openness of Rome’s roads, which connected her without fear to the world. These roads were like the shipping routes that had crossed the Aegean Sea when Athens was at her apex, which had brought the goods of the world to the foremost Greek city. The openness of Roman roads was true power, far stronger than mere walls. These roads were lines in the face of a confident city, the sinews of an invincible civilization with a people who admitted to no threats capable of striking their capital. Now the threats were real, and a new invasion was only a matter of time. Aurelian’s walls might have reassured the Romans that another invasion could be held off before penetrating the city itself, although Rome remained dependent on outside sources for its food and water. But the walls may also have had a deeper effect: they could not save Rome, but they could remind every Roman, every day, that he was perpetually at risk. The assault of the Goths onto Italy had been shocking but temporary evidence of Rome’s precarious position-as the sight of the Thebans had revealed the paucity of the Spartans, and Scipio’s landing swept fear through Carthage-but Aurelian’s walls were an open admission of permanent weakness and vulnerability. If any doubt remained that Rome was no longer the source of energy at the center of empire, that doubt was now removed. If one wished to name a particular event as marking the true Fall of Rome, one might look not to the elevation of King Odoacer to emperor in 476, or to the sack of Rome in 410, or even to the division of the empire in 395. The moment at which the city of Rome became just another town among many in an anarchic landscape, and not a source of law and power, was AD 271, with the decision to build walls around Rome. The political and military center of the empire had been gutted; the rest was a matter of time.”

Read John David Lewis’s book, “Nothing less than Victory”, for more info regarding walls erected in ancient times to protect the cities.