Grado's historic core is dominated by this Romanesque basilica, dating to AD 579. Its rich interior decoration indicates its early importance when the church became the seat of the Patriarch after Attila's hordes drove the citizens of Aquileia onto the island. Roman marble columns line the nave and frame the frescoed apse, while the floor is decorated with a 6th-century, Byzantine-style mosaic.
The adjoining lapidary contains 4th- to 5th-century mosaics as well as some 3rd-century Roman sarcophagi facades.