A Renaissance masterpiece, the duomo's cupola – 91m high and 45.5m wide – was built between 1420 and 1436. Filippo Brunelleschi, taking inspiration from the Pantheon in Rome, designed a distinctive octagonal form of inner and outer concentric domes that rests on the drum of the cathedral rather than the roof itself. Four million bricks were used, laid in consecutive rings according to a vertical herringbone pattern. Advance reservations, online or at the cathedral's Piazza di San Giovanni ticket office, are obligatory.
When Michelangelo went to work on St Peter's in Rome, he reportedly said: 'I go to build a greater dome, but not a fairer one'. The cupola crowning the duomo is a feat of engineering and one that cannot be fully appreciated without climbing its 463 interior stone steps.
The climb up the spiral staircase is relatively steep, and should not be attempted if you are claustrophobic. Make sure to pause when you reach the balustrade at the base of the dome, which gives an aerial view of the octagonal coro (choir) of the cathedral below and the seven round stained-glass windows (by Donatello, Andrea del Castagno, Paolo Uccello and Lorenzo Ghiberti) that pierce the octagonal drum.
Look up and you'll see flamboyant late-16th-century frescoes by Giorgio Vasari and Federico Zuccari, depicting the Giudizio Universale (Last Judgement).
As you climb, snapshots of Florence can be spied through small windows. The final leg – a straight, somewhat hazardous flight up the curve of the inner dome – rewards with an unforgettable 360-degree panorama of one of Europe's most beautiful cities.
It is impossible to visit the cupola without an advance reservation, which can be made online or at self-service Ticketpoint machines located inside the Duomo ticket office, opposite the main entrance to the Baptistry at Piazza di San Giovanni 7. Book at least a month in advance in high season; in the low season, a couple of days in advance.