Romans used paid mercenaries to maintain the last few centuries. And I believe they paid Attila to stay away for a few years. The remarkable thing as you say was one city doing this for so long. But they had a network of help
They didn't have a network of help, they built one. Their use of selective trade (bribery) to co-opt neighboring tribes along its borders, and bringing non-Roman areas into the Roman economy worked very well until a series of depopulation events within the Empire that may have been the first big instance of the Black Plague. One reason Augustus cut the Roman regular army in half after the defeat and death of Antony was to right-size Roman forces and bring legions up to strength; a second reason was to free up Romans to get back to their regular lives and to cut the military budget; yet another reason was, the Empire had grown quickly and ruled many more non-Roman people.
The auxiliary legions were derived from conquered people who had their own traditional fighting styles and techniques, and they were deployed in parts of the Empire distant from their old homelands (making them cleave to their Roman commanders and regular legions); as Roman citizenship was continually expanded, Romanized people of non-Roman backgrounds increasingly made up the regular army.
The first non-Roman/provincial Emperor ascended to the big chair late in the 1st century (Trajan, born in Iberia but of non-Roman Umbrian stock) and he was a real ass-kicker. OTOH, Septimius Severus had Carthaginian roots, and his rule and those of his sons preceded what is generally called the Disaster of the Third Century (again, probably plague-induced).
One of my favorites, Aurelian, was born to a non-Roman family in Dacia; on his first outing he was defeated at first by barbarian invaders, who decided to split up to sack and plunder separately while Aurelian rode around and gathered his scattered troops; when Aurelian struck, he smashed the smaller groups of barbarians one by one until there were no more. He built the Aurelian Wall around Rome (parts are still visible today), knocked out the pretenders around the Empire, and put the whole works back together, just in time to be assassinated by an embezzler on his staff who thought he'd been found out. A ruled about five years.