You would recognize the general topography of the city and a few landmarks. Virtually everything else, however, would be strange.

In 133 BC, the center of Rome was just beginning to assume the grandiose appearance we associate with it. The Forum had a few substantial temples and basilicas, the Capitoline Hill was crowned by the venerable but still impressive Temple of Jupiter, and the Palatine had sprouted a gaggle of imposing aristocratic houses. But with the exception of some scattered temples, the rest of the city was virtually devoid of monumental architecture.

In the half-millennium between 133 BC and AD 400, the center of Rome was transformed (speaking very generally) by three factors: a series of disasters, the rise of unimaginably wealthy patrons, and (at the end of the period) the advent of a new religion. To take these in turn...

The most famous disaster to transform the face of Rome was of course the great fire of 64 (the one Nero "fiddled" through), which devastated 10 of Rome's 14 districts (Tac., Ann. 15.40). This, however, was only one of many conflagrations. Another devastated much of central Rome in the reign of Domitian, and yet another shortly before the reign of Diocletian - and these are only the ones our sources see fit to mention. As a result of these fires (and the earthquakes and floods to which Rome was so lamentably prone), virtually every public building standing in 133 BC had to be rebuilt at least once over the course of the imperial era.

Even more transformative was the second factor I mentioned: the rise of unimaginably wealthy (and, to a man of 133 BC, unimaginably powerful) patrons. The first of these were the late Republican generalissimos - above all, Pompey and Caesar, who each sponsored massive building projects. The emperors followed suit. Augustus famously rebuilt much of Rome in his own image:

"Since the city was not adorned as the dignity of the empire demanded, and was exposed to flood and fire, he [Augustus] so beautified it that he could justly boast that he had found it built of brick and left it in marble." (Suet., Aug. 28)

Several other first- and second-century emperors were great builders: Vespasian and Titus built the Colosseum; Trajan, the greatest of the Imperial Fora; Hadrian, the colossal Temple of Venus and Rome; Caracalla, some really swanky baths, etc. The cumulative result of their efforts was to transform the Forum into a museum of imperial splendor, and to seed monumental buildings through many of the adjacent districts (especially the Campus Martius).

Finally, and belatedly, Rome became Christian. Since the first monumental churches were built on the outskirts of the city, relatively little would have changed in central Rome by the year 400. But the transformation was visibly, if subtly, underway: the statue of Victory, for example, had already been removed from the Senate house at Christian insistence.

To a late Republican time traveler, late antique Rome would be an incredibly splendid place, filled with monumental buildings that dwarfed those of his own time. The Colosseum and the Pantheon, in particular, would have been wholly outside his architectural experience. A few buildings would be familiar; in the Forum, for example, the Temples of Saturn and of Castor would be in their expected places, though both in reconstructed form. The great temple of Jupiter on the Capitoline would likewise be both familiar and strange, having been reconstructed with spindly marble columns and a gilded roof in the reign of Domitian. Our time traveler, in short, would recognize the outlines of his city; but he would also recognize that the politics and society of that city had changed profoundly.