The façade, in Romanesque-Gothic style, is connected by a portico to the so-called "Church of the Pagans", and the remains of the 5th-century baptistery. The interior has a nave and two aisles, with a noteworthy mosaic pavement from the 4th century. The wooden ceiling is from 1526, while the fresco decoration belongs to various ages: from the 4th century in the St. Peter's chapel of the apse area; from the 11th century in the apse itself; from the 12th century in the so-called "Crypt of the Frescoes", under the presbytery, with a cycle depicting the origins of Christianity in Aquileia and the history of St. Hermagoras, first bishop of the city.