Basilica di San Giovanni in Laterano

Despite St. Peter's Basilica being far more famous, the Archbasilica of St. John Lateran is actually Rome's cathedral — the seat of the Pope in his specific role as Bishop of Rome — and holds the formal title 'mother and head of all churches in the city and the world,' inscribed directly on its facade in Latin. Founded by Emperor Constantine in the early 4th century, it's the oldest of Rome's four papal basilicas and, for over a thousand years until the Avignon Papacy relocated the papal seat in the 14th century, was the primary papal residence and centre of the Catholic Church, not the Vatican.

The current facade and much of the interior date to later renovations, most dramatically an 18th-century Baroque remodel by Alessandro Galilei, but the basilica retains extraordinary older elements: a bronze door reputedly taken from the ancient Roman Senate house (the Curia Julia in the Forum), and — across the street in a separate building — the Scala Sancta, 28 marble steps traditionally believed to be from Pontius Pilate's palace in Jerusalem, which pilgrims still climb on their knees as an act of devotion.