History

The site of the Roman Forum was originally a marsh between the Palatine, Capitoline, and Esquiline hills. Drainage by the Cloaca Maxima (7th century BCE, tradition assigns it to King Tarquinius Priscus) made the valley habitable; by the 5th century BCE, the Forum was the commercial and civic centre of the growing city. The Temple of Vesta and the adjacent House of the Vestal Virgins, the Basilica Aemilia (179 BCE), the Temple of Saturn (built 498 BCE, rebuilt 42 BCE, current ruins 4th century CE) — layers of construction over nine centuries have left the Forum as a palimpsest in which Republican and Imperial structures overlap and contradict each other at every step.

The Colosseum was built on the site of the artificial lake of Nero's Domus Aurea; Vespasian began it in 72 CE as a deliberate return of public land to the Roman people after Nero's expropriation. The building uses four different orders on successive storeys (Tuscan, Ionic, Corinthian, and Composite), a system that became the canonical facade grammar for public buildings across Europe from the Renaissance onward. The Colosseum hosted gladiatorial games, animal hunts (venationes), and public executions until 523 CE; thereafter it served as a quarry whose marble and travertine fed medieval Roman construction for centuries. The surviving structure is approximately two-thirds of the original building.

The Pantheon is the best-preserved ancient building in Rome because it has been in continuous use: Gregory III consecrated it as Santa Maria ad Martyres in 609 CE. Hadrian's reconstruction of the earlier Agrippan building (27 BCE) kept the original dedicatory inscription on the portico (“M. Agrippa L. F. cos. tertium fecit”) despite replacing the entire building: an act of architectural modesty remarkable for an emperor. The 43.3 m concrete dome was cast in a single continuous pour using pumice aggregate that grew progressively lighter toward the oculus, reducing the dome's weight at its thinnest point.