WE BEGIN OUR PROCESS OF UNTANGLING THE MEDIEVALIZING OF ROME with two urban pendants, a pair of images: Rome as it was two or three generations after the Milvian battle, in the late fourth century, and Rome in the twelfth century – a long time in the life of any organism, not least a great city. The point is to show the general structure of late-antique and late-medieval Rome, and to ask the question, How did we get from A to B? With what process, speed, and purpose did the city progress, over eight centuries, to a point where untidy and exuberant urban scrub had penetrated and overgrown the grand order of the imperial capital, without however being able to obscure it entirely (Fig. 109)?

There are two ways of studying this contrast between late-antique and late-medieval images of Rome. One is common, and the humanist poet and scholar Francesco Petrarca (1304–1374), known as Petrarch, was the first Renaissance writer to encourage it: to view the late-medieval city unfavorably against the ancient substance of the Constantinian city. His aim was political and propagandistic – to resurrect the notion of a resplendent classical Rome, which had been abused and belittled during the Middle Ages. He and later Renaissance writers assigned themselves the task of giving birth anew to this classical splendor. Medieval Romans themselves were not taken up with laments. It was their city and they lived in it, adjusting and reshaping, adding and curtailing, fitting it to their own lives. And that is the second way of looking at the contrast, to study the vital processes that led from the mighty metropolis of the Mediterranean to the different and equally viable city, the Seat of Peter. We must avoid the mistake of seeing the thousand years of medieval Rome as an unchecked slide from Trajanic or Hadrianic summits toward physical degradation and collapse. Our goal is to understand medieval Rome, not to condemn it.

How can we account for a city that during the fourth century was essentially whole but became disjointed, environmentally and socially – an assembly of more or less independent parts fortified and pitted against each other in the twelfth century?