[Jeff Chandler]

When they started building walls around Rome, the Roman Empire was done for.

Yep. Another 800 years (counting the republic) and they were gone.

[MUDDOG]

I'm talking about the Aurelian walls (ca. 270 AD). The original walls dated from about 400 BC, but Rome didn't need walls from about 200 BC (after Hannibal) until the crisis in the third century AD, and from 200 BC until then expanded outside the original walls. Rome was renowned for not depending on walls, depending on its army to defend iself.

I believe Gibbon observed that the construction of the Aurelian walls was a sign of decline.

[MUDDOG]

Here's from Gibbon ("Decline and Fall of the Roman Empire") re the construction of the Aurelian walls:

"It was a great but melancholy labour, since the defense of the capital betrayed the decline of the monarchy. The Romans of a more prosperous age, who trusted to the arms of the legions the safety of the frontier camps, were very far from entertaining a suspicion, that it would ever become necessary to fortify the seat of empire against the inroads of the barbarians."