The Cathedral of Santa Maria del Fiore is Florence's undisputed centerpiece, a building so audacious in scale and ambition that it took nearly 150 years to complete. Construction began in 1296 under Arnolfo di Cambio, but it was Filippo Brunelleschi who delivered the masterstroke in 1436 — an eight-sided brick dome that remains the largest masonry dome ever built. No one had solved the engineering problem of how to span the 44-metre gap without conventional wooden centering, and Brunelleschi essentially invented the techniques as he went. Standing beneath it, that fact becomes viscerally real.
The exterior is a theatrical patchwork of white Carrara marble, green marble from Prato, and pink marble from the Maremma — a polychrome cladding added in the 19th century that many architects have called garish and tourists tend to find irresistible. The interior, by contrast, is sober and vast, stripped of the gilded clutter you find in Rome or Venice. What you see is Gothic space and engineering confidence. Look for Giorgio Vasari and Federico Zuccari's vivid Last Judgment fresco ringing the inside of the dome, and the two equestrian fresco portraits of mercenary commanders — a peculiar, secular flourish in a sacred building.
Climbing the dome is one of the great physical experiences in European travel. The ascent winds through the double-shell structure Brunelleschi designed, up 463 steps with the brickwork inches from your nose, before emerging on the lantern terrace with a 360-degree panorama over Florence's terracotta roofscape. Book the dome climb separately and well in advance — it sells out days ahead, especially in summer. The separate Giotto's Campanile offers an arguably better view of the dome itself for those who want the photograph rather than the interior perspective.
A single Duomo Complex ticket covers the cathedral, dome, campanile, baptistery, crypt, and museum — the Museo dell'Opera del Duomo houses the originals of many works now replaced by replicas outside, including Ghiberti's astonishing Gates of Paradise. Go to the museum last; after you've seen the building itself, the objects inside acquire considerably more weight. Arrive at the cathedral just after opening or in the final hour before closing to avoid the worst of the crowds, and note that entry to the cathedral interior itself is free — only the climb and the complex ticket cost money.