The mighty St. Gallus church from the 17th to 18th century sits enthroned above the city. The church was constructed on a pre-Christian cult site and was redesigned on multiple occasions by the famous baroque master builders of the Bregenz Forest.
The church was built to the site of pre-Romanesque church founded around 450 AD. Irish monks Columbanus and his brother Gallus founded the destroyed church in 610 AD and ordered to rebuild it.
The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.