Nearly six centuries after it was completed, the dome of Santa Maria del Fiore cathedral in Florence—a cathedral known around the world simply as il Duomo—remains that city's icon and greatest pride. Built without flying buttresses or freestanding scaffolding, using experimental methods that many contemporaries believed would surely fail, the 150-foot-wide (46-meter-wide) dome effectively ignited the creative explosion known as the Renaissance.

Its creator, Filippo Brunelleschi, a homely, hot-tempered goldsmith with no serious architectural training, is a hero to his fellow Florentines—and to one modern architect in particular. (Read more about il Duomo in the February 2014 issue of National Geographic magazine.)

Last April 15, on the 567th anniversary of Brunelleschi's death, I stood in the portal of the Florence