The plan of the building in the form of a Latin cross is probably due to the renovation carried out by the Sienese architect Anton Maria Lari, subsequently updated stylistically after the baroque modifications, between 1858 and 1865. The current aspect of the central aisle is the result of the sixteenth and nineteenth-century re-workings.
Between 1592 and 1662 the vaulted ceiling replaced the preceding wooden trussed roof. The nineteenth-century restoration work directed by the architect Fabio Nuti included the realisation of a low dome at the intersection of the transept, decorated with neo-Classical paintings and contrasting stone facing of the pilasters and arches. The baroque altars collocated along the walls of the lateral aisles were removed.
The apse at the end had been renovated in the sixteenth century in a polygonal shape, thus allowing space for a choir and two chapels in a radial arrangement, and after the restoration had remained incomplete. In the XVII century it was restructured in its current semi-circular form, providing access to the new sacristy. In the apse there is a painting of Rustici representing the saints Carlo Borromeo and Lorenzo in the act of adoring the name of Jesus.
In 1649 the great altar was realised in coloured marble, designed by Tommaso Redi and completed with ciborium in 1692 by Giovanni Antonio and Francesco Mazzuoli. On the front there is an antependium in silver and gilded copper, carried out in 1782 by the Sienese Giacomo Bonechi for the altar of the 'Madonna delle Grazie'.
In the Chapel of the "Madonna delle Grazie" on the left side of the transept, there is a painting depicting 'La Vergine Assunta con angeli' ('Our Lady of the Assumption') realised around 1470 by the painter Matteo di Giovanni, probably the central part of a larger painting. On the right side of the transept there is the Chapel of the Crucifix, built in the mid-nineteenth century. It contains a wooden cross of the second half of the XV century, perhaps work of the 'Vecchietta'. The altar was built in imitation of the one in the other chapel in 1857 by the sculptor Domenico Iardella from Lucca.
On the other façade above the central portal there is a tympanum sculpted with the figure of Christ in a ‘mandorla’ held up by angels, and two column-bearing lions attributed to Giovanni d'Agostino. The stained-glass windows were made by Girolamo di Benvenuto in about 1470. In the central aisle there is a richly decorated holy-water font composed of a shaft bearing an inscription with the date 1506 and name of the donator Gerolamo Vantaggioli, and an ornate basin perhaps made by Antonio Federighi. The baptismal font collocated in the left aisle is by Antonio Ghini, responsible also for the altar of the Chapel Madonna delle Grazie, 1470-1474. In the ante-sacristy there is a marble ciborium from 1500 that originally came from Castiglione della Pescaia.