Sienese architect Arnolfo di Cambio began work on it 1296, but construction took almost 150 years and it wasn't consecrated until 1436. In the echoing interior, look out for frescoes by Vasari and Zuccari and up to 44 stained-glass windows.
The Duomo's neo-Gothic facade was designed in the 19th century by architect Emilio de Fabris to replace the uncompleted original, torn down in the 16th century. The oldest and most clearly Gothic part of the cathedral is its south flank, pierced by Porta dei Canonici (Canons' Door), a mid-14th-century High Gothic creation (you enter here to climb up inside the dome).