The fifth hour of the night, and the pavement still blood hot. The darkness is torrid too, with the dusty heat of late Augustus. A crimson moon bleeds through lines of tenements.

You are quite drunk and - all things considered - reasonably happy. It was a wonderful banquet (or at least, the wine was wonderful), and you managed to collect a fine assortment of leftovers in your linen napkin. You hadn't meant to stay so long; but with the night so hot, why bother trying to sleep?

You look around blearily. Which street is this again? They all look so similar in the dark, and most of them are so dark after nightfall. They all have filthy pavement and filthier gutters. They all have overhanging tenements and lines of shuttered shops. The same crossroads shrines, the same stray statues, the same shadowy knots of men, watching from alleyways.

You stop. Were those footsteps behind you? Or just the echo of your own from the stuccoed walls on either side? You start walking, faster, then stop again suddenly. A moment too late, the second footsteps stop too. Suddenly aware of the heat, you begin to trot as fast as you sweaty toga will allow. At night, there are worse things than rats on the streets of Rome...

Apologies for all the literary color. To get briskly nonfictional: if we can trust our sources, violent crime was always a problem in Rome, but never associated with any single part of the city. As I've written in a few answers, certain parts of Rome - above all, Trastevere - were associated with foreigners. Others, such as the Subura, were regarded as seedy. Others still, such as the shanty suburbs among the tombs beyond the city limits, were perennially poor. There were, however, no "ghettos" in the modern sense, for a few reasons.

First, Rome - which had nothing like modern zoning - was never as wealth-segregated as many modern cities. The mansions of the wealthy were scattered throughout the city, and often sandwiched between low-rent apartment buildings. Julius Caesar himself was a longtime resident of the Subura.

Second, and as importantly, footpads and murderers were - to believe Juvenal, anyway - everywhere in Rome after dark. Like most ancient cities, Rome was more or less innocent of street lighting. If one had to venture out after dark, it was considered advisable to do so with at least one companion. Wealthy banquet guests were customarily escorted to and from home by retinues of torch-bearing slaves. Rome's vigiles (night watchmen) were primarily firemen, not police, and do not seem to have tamped down the crime rate with any real vigor or success.

But if all of Rome was dangerous after dark, no part of the city was notoriously dangerous in daylight. Rome was an intensely public place, its streets packed with shoppers and strollers and idlers. Plenty of petty theft, of course, took place despite all those watching eyes. Yet outside the turbulent years of the late Republic, when political disturbances convulsed the city, violent crime seems to have been relatively uncommon.

Modern cities are of course products of distinctively modern (industrial / post-industrial) economic systems, means of transportation, and social anxieties. There was a great deal of crime of Rome. But it never seems to have been localized in the ways we have been conditioned to expect.

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The most comprehensive single-volume account of the city of Rome in antiquity is Stephen Dyson's Rome: A Living Portrait of an Ancient City. My favorite handbook of daily life in Rome is J. V. P. D. Balsdon's Life and Leisure in Ancient Rome.