Nostra Segnora de Mesumundu is located in the archaeological complex with the same name. It was built in the 6th century, during the Byzantine domination of the island, over a pre-existing Roman structure (2nd century AD). The Byzantines re-used part of the walls of the Roman building, as well as the aqueduct. The edifice could have been used as a baptistery. However, it is also possible that it was used for the purification of ill people through an immersion rite.
In 1063, the structure was donated by the Giudice (duke) Barisone I of Torres to the Abbey of Montecassino. When monks came from the Italian religious community, they adapted the building for Roman Catholic use, adding an apse and a new entrance (demolished in 1934). For the work, they used materials from the nearby Roman ruins and the nuraghe Culzu.
References: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.