The Cloaca Maxima — the Great Drain — is among the oldest continuously functioning pieces of Roman infrastructure. Built initially in the sixth century BC to drain the marshy valley between the Capitoline and Palatine hills that would become the Roman Forum, it has been carrying water to the Tiber for over 2,600 years. Tourists floating on the Tiber can still see its outlet — a rounded arch of tufa stone nearly four meters high, set into the river embankment near the Forum Boarium — and the drain itself, though substantially rebuilt and extended over centuries, remains active as part of Rome’s modern sewer and stormwater system. It is one of the oldest pieces of civil engineering in continuous use anywhere in the world.

The Colosseum was not called the Colosseum when it was built. Its official name was the Flavian Amphitheater — the Amphitheatrum Flavium — after the dynasty that commissioned and completed it. The name we use derives from a colossal bronze statue of Nero that stood nearby, a work of imperial self-aggrandizement that survived its subject by centuries. The building itself is formally anonymous, which is fitting for a structure whose purpose was to direct attention outward, toward the spectacle it contained, rather than inward toward the men who paid for it.

Isis arrived in Rome over official objections. The Roman Senate banned her worship multiple times in the first century BC — in 58, 53, 50, and 48 BC, with varying degrees of enforcement — ordering her altars demolished and her images removed from the city. The bans failed because the cult’s appeal was stronger than the official resistance, and by the first century AD the goddess who had been repeatedly expelled was being worshipped in temples funded by emperors. Caligula built her a major temple in the Campus Martius. Vespasian and Titus celebrated their triumph over Judaea in her temple precinct. Commodus appeared in her processions in priestly dress. Whatever the Senate of the Republic had thought about Egyptian divinities, the Empire had reached different conclusions.

Rome was not original in its theology, and it did not pretend to be. The Romans were systematic borrowers of divine power, operating on the practical assumption that a god who worked was worth incorporating regardless of origin. The result was a pantheon that was Greek at its core, overlaid with indigenous Italian tradition, supplemented by imports from Egypt, Persia, and Syria, and eventually contested and replaced by a monotheism that originated in Judea. Roman religion was an accumulation, not a creation.

Rome fed a significant portion of its population for free, and had been doing so, in various forms, for over five centuries by the time the Western Empire collapsed. The grain dole — the frumentatio in its Republican form, the annona in its more developed imperial incarnation — was not a welfare program in the modern sense, though it served some of the same social functions. It was a political institution, a mechanism for managing the relationship between the imperial government and the volatile urban population of the capital, and it was expensive enough, logistically complex enough, and politically significant enough to have shaped the development of Roman administration, agriculture, and provincial policy for centuries.

The city of Rome at its height had a population of somewhere between half a million and a million people — the estimates vary and the ancient census figures are difficult to interpret — compressed into an urban area that had no master plan, no grid, and no effective building code until the fires that made such codes politically possible. The vast majority of these people lived not in the marble houses of imperial imagination but in multi-story apartment buildings called insulae — islands — so called because they filled city blocks the way islands fill water, surrounded on all sides by streets. The insula was Rome’s residential reality, and it was often dangerous, frequently squalid, and occasionally catastrophically flammable.

The Pantheon is the best-preserved ancient building in the world, and it is better preserved than most medieval buildings, because it has been in continuous use since its construction. Hadrian built it between approximately 118 and 128 AD on the site of earlier temples in the Campus Martius district of Rome, and it has served as a temple, a church, a tomb, and a tourist site across nineteen centuries without the structural interruption of abandonment. The dome that spans its interior has not been surpassed in diameter — 43.3 meters — by any unreinforced concrete construction in the two thousand years since it was poured. Whatever Rome’s engineers knew, they knew something that took a very long time to recover.

The Praetorian Guard killed four emperors, elevated at least five more to power, and constituted the single most politically destabilizing institution in Roman imperial history. This was not a design intention. Augustus established the Guard as a personal security force — a professional bodyguard organized on military lines and stationed near Rome — because the emperor needed reliable protection and the Republic’s tradition of civilian governance had made no provision for one. What Augustus created as a security measure, his successors inherited as a power center whose loyalty could be purchased, whose commanders accumulated enormous influence, and whose physical proximity to the emperor gave it an influence over succession that no amount of constitutional theorizing could override.

Rome did not conquer its empire and then build roads to administer it. The roads and the conquest advanced together, each enabling the other in a feedback loop that eventually produced the most extensive road network the ancient world had ever seen. At its peak, the Roman road system covered somewhere between 250,000 and 400,000 kilometers, of which roughly 80,000 kilometers were stone-paved primary roads capable of moving legions, supplies, and official communications at speeds that would not be matched in Europe until the nineteenth century.

The Roman bath was not primarily about hygiene. That framing, which modern people find intuitive, misses what made the baths central to Roman urban life for centuries. The bath was a social institution — a place where Romans of different classes shared the same water, the same space, and the same several hours of the afternoon in an arrangement that had no precise equivalent before or since. It was the forum, the gym, the library, the barbershop, and the social club compressed into a single building and made available, often for free or for a nominal fee, to virtually everyone in the city.