According to tradition, the church is one of the 100 churches founded by Greek missionary Saint Giulio (4th century). Located in Crusinallo, the most densely populated area in Omegna, the Romanesque church comprises two unequal naves supported by granite columns. The foundations of the first church and the bell tower basement suggest that the structure was built in the 11th century. The asymmetrical façade maintains its Romanesque charm despite the signs of the subsequent extensions and changes: during the first half of the 17th century, a second nave with rib volts was added to the first nave and apse. The pronaos, the ancient chapel-ossuary demolished after the First World War and the baptistery attached to the bell tower – which hosts an anonymous fresco of the baptism of Christ – are of the same period. The Romanesque base of the bell tower was risen in the first half of the 19th century and a new bell chamber with five bells was added. Inside, visitors can admire: the main altar, made of multi-colour marble with balustrades, which dates back to the 18th century and is dedicated to Saint Gaudenzio; the beautiful polyptych in the apse, by Filippo Cavalazzi (1567); the chapel of Our Lady of the Rosary, with marble balustrades; and the wooden statue of the Virgin (18th century). The baptismal font, by Jacob de Albo, dates back to 1612, which is also when the baptistery and the old sacristy – which became the third nave after 1972 – were built.
Next to the church, on a rocky hill, is the Oratory of Saint Rocco (1608), also known as the “Church of the Castle”. The road that leads to the Castle is dotted with chapels marking the Stations of the Cross. The church was built on the ruins of an old medieval tower, built by the Counts of Crusinallo, a branch of the "da Castello" family. The town later dedicated the church to Saint Rocco, who protected it during epidemics.