The town of Castelsardo originates from the castle built in the 12th or 13th century by the Doria family of Genoa. The castle and the village which gradually formed round it where the seat of the Doria's fiefdom in the island called Castel Doria or Castelgenovese, until it was conquered by the Aragonese in the 15th century (1448), and named Castillo Aragonés (Castel Aragonese). Later it became the seat of the bishopric until the Cathedral of Sant’Antonio Abate was built in 1586.
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.