AUGUSTUS’ SUCCESSOR, TIBERIUS (14–37 C.E.), SPONSORED ONLY A FEW urban developments in Rome, the most significant being the expansion of the imperial Palatine compound into the Domus Tiberiana, a genuine palace designed to accommodate the growing imperial entourage, especially the Praetorian Guard. Other cohorts of the guard were quartered in a full-scale military camp, the Castra Praetoria, just northeast of the Servian Wall; this was later nested into the Aurelian Wall (see Fig. 80). Caligula (37–41) launched several aggressive building campaigns, some of which died prematurely with their patron. Others were completed by his successors. Claudius (41–54) focused on infrastructure rather than the city's monumental core. The splendid Porta Maggiore, doubling as an aqueduct arcade and a city gate, is his most enduring monument in Rome (Fig. 38). Over it flowed his two great aqueducts, one channel stacked upon the other: the Aqua Claudia and the Anio Novus above, both begun by Caligula. Their arcade of solid peperino stone still dominates the landscape east of the city. Together they augmented the existing water supply by as much as 40 percent. Claudius devised no known monumental baths, water play, or naumachia to justify or advertise them. Yet combined, Frontinus later tells us, the two lines were supplying all 14 city regions from 92 distribution tanks in his time (ca. 97 C.E.). In number, these tanks constituted 37 percent of the total urban network, and in volume 34 percent; the total brought in an astounding (but in Frontinus’ opinion, scandalously underperforming) 333 million liters per day.

This precious glimpse of Rome's “soft” structure at one moment in its history says little about its “hard” aspect, however. After Augustus, the first traceable and truly systemic changes to the urban fabric belonged to Nero (54–68), who took concrete vaulted architecture not only to new heights of creativity, but also into mass production. The central-Italian preference for concrete as a monumental building material developed slowly and incrementally. Writing early in Augustus’ reign, Vitruvius provided a simple formula for this miraculous slurry that hardened into veritable stone, far stronger than any mortar. Its key ingredients were chunks of stone aggregate and a matrix of water, slaked lime, and pozzolana – a volcanic sand that gives concrete its extraordinary strength.