IF MEDIEVAL ROME, IN ITS LONG HISTORY, HAD ANYTHING LIKE A MORAL and administrative focus, it was the Lateran palace and the basilica Constantiniana, now called S. Giovanni in Laterano, St. John Lateran, which Constantine, freshly victorious at the Milvian Bridge, built to serve the pope, bishop of Rome. From that moment the fate of medieval Rome was bound to this remote corner of the city, which grew steadily in girth and importance even as the inhabited core contracted away from it. By the ninth century the Lateran complex had become to papal Rome what the Palatine had been to imperial Rome, the supreme seat of administration and political power. Despite ravaging invasions and other adverse conditions, despite the search for alternative seats of papal rule, the Lateran remained inarguably preeminent until the official move to the Vatican in 1420 following the return of the papacy to Rome.

Our investigation into the Lateran's first 1,100 years must be brief, and sadly there is little left to see of this once-magnificent theater of autocratic rule. Other than the baptistery, some adjacent structures, the rebuilt and relocated Triclinio Leonino, and the largely thirteenth-century Sancta Sanctorum, new construction between the sixteenth and nineteenth centuries devoured or covered over nearly the entire medieval scheme. Still, the Lateran was relatively lucky. In an exhaustive study of the physical and literary documents that Rohault de Fleury published in 1877, we have a compelling plan and reconstruction drawing of the complex at the end of the Middle Ages, as it stood in 1300 – the first jubilee year, which drew Giotto and Dante to Rome – pristine and isolated from its environment (Figs. 125, 126).

But because our environmental approach privileges process over the culminating product, our task is to peel back the layers of time so that we can see the matrix of streets, the Aurelian Wall, aqueduct arcades, and other elements that spawned a whole urban quarter we call, by the common Italian term for such a compound, the Lateran Borgo. To aid our reconstructive surgery, Rodolfo Lanciani's Forma Urbis Romae (1893–1901), which shows the modern streets and buildings over the medieval and ancient Lateran neighborhood, is essential (Fig. 127).