Catedrale di Santa Maria (Duomo)

Consecrated by Pope Alexander III (from Siena) in 1189. In 14th century, arches were constructed as part of a plan for a much larger cathedral. The plans were abandoned due to structural problems and the Black Plague, which killed 4/5 of Siena's population. You can climb stairs to the top of the arches for a good view of the city center (from the Museo dell'Opera del Duomo).

Siena's cathedral, the Duomo, begun in the 12th century, is one of the great examples of Italian gothic architecture. Its main facade was completed in 1380. Its campanile and baptistry make a fine group. Inside is the famous Gothic octagonal pulpit by Nicola Pisano (1266–1268) supported on lions, and the labyrinth inlaid in the flooring, traversed by penitents on their knees. Beneath the Duomo, in the baptistry is the baptismala marvellous font with bas-reliefs by Donatello, Ghiberti, Jacopo della Quercia and other 15th-century sculptors. The Museum of the Opera del Duomo contains Duccio's famous Madonna (1308–1311).

Basilica di Santa Maria dei Servi

Consecrated in 1533. Lovely Renaissance interior and somewhat creepy relics – two altars have glass display cases of saints, one a skeleton with wax head and hands, the other a mummified body. Located on the hill, the church affords a great view of Siena.

Sant'Agostino Church

Built over a 200-year period from 1258 to late 1400s. Severely damaged by fire in by fire in 1747 and restored by 1755. Attached to Augustinian monastery.

Chiesa di San Niccolò al Carmine (Church of St. Nicholas)

Was once a Carmelite convent; now houses university institutes. Religious services are held in the church before the horse races each summer.

Basilica of St. Francis

Contains miraculous consecrated hosts that have remained intact since 1730. The full story of the miracle can be read here.