The Saint Donatus church (Sv. Donat) in Zadar (Croatia) is a beautiful Pre-Romanesque / Byzantine (9th century) church.
The massive dome of the rotunda is surrounded by a vaulted gallery in two stories which also extends around the three apses to the east. In the foundations that are partially open to see one can spot parts of various columns that used to be part of the old Roman forum.
This lead to the traditional belief that this church was built on the site of a temple of Juno (though that is now disputed and believed to be a wrong conclusion).
Location: Zadar (Croatia)