Time has hidden the traces of the original church built by St Giulio, that, legend has it, was on the highest rock. A simple chapel was constructed in the 5th-6th centuries where the basilica is now (orientated unusually to the north) but was replaced by the present church in the 12th century. The front is orientated towards the west and its Romanesque façade can be seen from the lake but this door is no longer in use –the west banks of the island are unapproachable when the tramontana wind from the north churns the lake to a maelstrom. The entrance is now from the southern door, up the steps from the landing stages.
A typical basilica layout, with nave, aisles and a gallery, the twelfth century basilica has been heavily re-managed during the centuries and over-adorned with gilded angels and baroque curlicues. There are some fine 14th-16th century frescoes and the unique carved Romanesque pulpit (or ambone), a work of the highest art. To the right of the present entrance is the sacristy. Under the first pillar as one enters is the marble sarcophagus of Mimulfo, now used to collect offerings. Below the altar is a crypt with a silver urn containing the supposed remains of St Giulio, silver-masked and finely clad in bishop’s robes. A doorway at the back of the crypt leads to a tiny museum where fragments of the earlier church are on view.