The cathedral is shaped like a Latin cross, with a central nave terminating in an apse and four side aisles and a transept of three aisles.
The central nave is flanked by monolithic granite pillars with 11th and 12th century capitals. A women’s gallery runs above, with triple mullioned openings that continue around the transept. The eliptic dome, with a fresco of the Virgin in Glory and Saintsby Orazio Riminaldi (1627-1628) strides the central span. The gilded coffered ceiling over the nave, transept and presbytery was carved by Domenico and Bartolomeo Atticciati between 1596 and 1602.
By the transept pilaster is the splendid pulpit by Giovanni Pisano (1302-1310). It is octagonal in shape, with nine plutei or panels carved with stories from the life of Christ and the Virgin and the Last Judgement, resting on ledges with Prophets and Sybils and carved capitals, and statues symbolizing the Virtues of the Church.
Admirable too, is the large mosaic in the apse, of Christ blessing with the Virgin and St. John the Evangelist, the work of Francesco and Lapo, with St. John’s face completed by Cimabue in 1302. Among medieval works are the 13th century Madonna di sotto gli organi, attributed to Berlinghiero Berlinghieri from Volterra, a fresco of the Madonna and Child by the San Torpè Maestro, on the triumphal arch and the Cosmatesque marble inlay floor (mid 12th century).
There are many valuable frescos, paintings and 16th to 17th century furnishings on the side altars ad in the presbytery.