The late 16th century design by Volterra for the church interior copied the medieval structure of six bays with a nave, two side aisles, an apse and a narthex. The nave and side aisles, preceded by a entrance vestibule, are divided by eight pillars supporting round arches, beyond which are three chapels on each side. The rich decoration is from the restoration of 1861 undertaken by the painter Cesar Mariani. He completed the work in 1866.