Exterior

The church is preceded by a 13th-century portico, in whose architrave was once a mosaic frieze with gilded letters. Today only a few fragments of the latter can be seen. Two central piers support a rounded arch, in which is the writing “Gloria in excelsis Deo…”, while another inscription at the top contains the name of the portico's designer, "IACOBUS CIVIS ROMANUS" ("James, citizen of Rome") and the date of execution (1210). The central and final piers of the portico are decorated with the marble symbols of the Four Evangelists, while in the middle of the arch is a lamb, symbolizing Jesus Christ.

The cathedral has three entrance gates. The central one is sided by four Corinthian columns, two of which stand on marble lions. Under the right lion is the representation of a child, a symbol of the faithful entering the church. The lunettes over the side gates have Cosmatesque mosaic decorations; the right lunette has also the representation of the "Blessing Christ".

The cathedral has a Romanesque bell tower.

The high altar

Interior

The church has a single nave, with side chapels which communicate with each other. The nave ends in a transept which precedes the raised presbytery, under which is the crypt. In the transept's arms are two monumental Baroque altars: the right one is dedicated to the Madonna of the Light, and incorporates an 11th-century fresco; the left-hand altar is dedicated to the Madonna of the Rosary, and has a 16th-century canvas surrounded by 15 panels, depicting the mystery of the rosary.

One of the two 13th century parapets walled in the crypt

The presbytery is home to one of the most precious works of art in the church, the high altar. It consists of a 4th-century Roman-Palaeochristian sarcophagus with seven niches which are decorated by biblical episodes. At the end of the presbytery, over the wooden choir, is a canvas by Pietro Nelli (Annunciation), while at the sides are two frescoes of the Resurrection of John and the Martyrdom of Sts. John and Marcianus.

A door on the left side of the presbytery leads to the oratory of Mary's Heart, built as a chapel in the 14th century. This houses important medieval works including:

• several 12th-century frescoes depicting a Madonna with Saints, an Angel and other Saints
• the front section of an early Middle Ages (9th-century) sarcophagus with representations of hunting scenes
• two Cosmatesque parapets flanked by two lions, originally in the cathedral's presbytery, dated to c. 1231-1240.

Under the presbytery is the crypt, with cross-vaults, dating to the 12th century but renewed later. It houses two Renaissance marble ciboria, originally in the presbytery but walled here during the 18th-century renovation works.

Wikimedia Commons has media related to Duomo (Civita Castellana).