The Roman emperors are not abstract historical figures in Rome — they're addresses. Augustus built the Forum you walk through. Nero's palace is underground beneath the park you picnic in. Hadrian designed the Pantheon you enter for free and the villa you visit in Tivoli. Marcus Aurelius's column stands in Piazza Colonna where the prime minister's office is. In Rome, 2,000 years ago is this afternoon.
Augustus (27 BC-14 AD): "I found Rome in brick and left it in marble." Visit: Forum of Augustus (Via dei Fori Imperiali — free view from the railing, or €4 underground visit). Ara Pacis (Via di Ripetta, €13 — the Altar of Peace, carved 13-9 BC). Mausoleum of Augustus (Piazza Augusto Imperatore — reopened 2021 after 80 years, €4, book ahead). The Res Gestae: Augustus's autobiography carved in bronze (lost) — a copy survives on the temple of Ankara (Turkey). A replica is on the wall of the Ara Pacis museum.
Nero (54-68 AD): The emperor who "fiddled while Rome burned" (he didn't — he was at Antium and rushed back to organize relief). Visit: Domus Aurea (his buried golden palace, VR tour €16, weekends). The Colosseum stands where his artificial lake was — Vespasian drained Nero's private lake and built a public amphitheatre as propaganda. The name "Colosseum" comes from Nero's Colossus — a 30m bronze statue of himself as the sun god that stood nearby.
Hadrian (117-138 AD): The traveling emperor who rebuilt the Pantheon (free, daily), built Villa Adriana in Tivoli (€12, a 120-hectare dream palace), and was buried in what is now Castel Sant'Angelo (€15 — his mausoleum, converted to a papal fortress). Hadrian's Wall in Britain was also his project. The man built on 3 continents.
Marcus Aurelius (161-180 AD): The philosopher-emperor who wrote Meditations while fighting Germanic tribes on the Danube. Visit: The equestrian statue on the Capitoline Hill (the original is inside the Musei Capitolini, €16 — the piazza has a copy). The Column of Marcus Aurelius (Piazza Colonna — spiral relief depicting his Danube wars, modeled on Trajan's Column). Constantine (306-337 AD): The emperor who legalized Christianity. Visit: The Arch of Constantine (next to the Colosseum, free). Old St. Peter's was his project (demolished to build the current basilica). The Basilica of Maxentius (Roman Forum — the 3 surviving vaults are the largest Roman arches standing).