WHATEVER THE GENEALOGY OF ROME'S GREATNESS, IT IS NOT PREMISED on geography. The city's physical advantages are undeniably significant, but hardly peremptory; when all is said and done, the natural setting seems a poor match for such a glorious destiny. Ancient Rome had no natural seaport and never dominated Mediterranean trade or transport in the manner of Carthage, Rhodes, Syracuse, or Alexandria. Nor did it command a fabulously fertile hinterland. It enjoyed no natural resources of note except clay, tolerably decent building stone, several small springs, and (we may presume) some quickly depleted timberland. Its hills were defensible but its valleys marshy or flood-prone. In its favor, Rome stood near the middle of the bustling Mediterranean basin at an important intersection of land routes and the Tiber, the largest and most navigable river in the region. This was an important, if hardly decisive, catalyst for the city's rise. The city occupies the lowest viable location for a major settlement in the river basin. Along its final run to the sea, the Tiber's banks are low, unstable, and prone to shifting during heavy floods. The ruins of Ostia, the ancient port town at the river's mouth 25 km below Rome, tell a cautionary tale: it was gradually buried in alluvium over time and its northern district was washed away by the sidewinding current.

Between Ostia and Rome, no bridge could have kept a grip on the river's wandering banks. Only inland – at the cluster of hills by which the world knows Rome – can some stability be found. Here the Capitoline, Palatine, and Aventine Hills are nested around the outer edge of an easterly elbow bend in the Tiber (Fig. 1). River bends tend to amplify into loops over time, but here the barrier of hills thwarts this tendency, providing a short zone of equilibrium that permits bridging. The loop contains a stable island as well, a rarity for the Tiber. Yet the island provides no advantages for a permanent crossing; in fact, the earliest recorded bridges of Rome, the wooden Pons Sublicius and the stone Pons Aemilius, were both downstream from it.