It was the 1401 competition for the commission to produce the second set of Baptistery doors that marked the turning point.

WSJ: Review: The Flowering of the Florentine Renaissance | 'Springtime of the Renaissance: Sculpture and the Arts in Florence 1400�C1460,' at the Palazzo Strozzi in Florence

It was a Christian settlement in the 6th century, and its massive baptistery, one of the largest in the eastern Mediterranean after the baptistery of Hagia Sofia in Istanbul, still survives.

ECONOMIST: Mediterranean archaeology

Andrea Pisano, here with a marble relief panel from Florence's campanile depicting the Greek sculptor Phidias at work, also made the first set of bronze doors for Florence's Baptistery, between 1330 and 1336.

WSJ: Review: The Flowering of the Florentine Renaissance | 'Springtime of the Renaissance: Sculpture and the Arts in Florence 1400�C1460,' at the Palazzo Strozzi in Florence

In 1401, when he was just 24, Brunelleschi had gone after another prominent commission--for a set of bronze doors for the 7th-century Baptistery of San Giovanni, a striking octagonal structure just west of the cathedral.

FORBES: Dome of the Brave