Nicastro's origins trace back to the 9th century, when Calabria was part of the Byzantine Empire, when a fortress called Neo Castrum ('New Castle') was created. In 1057, the area was conquered by the Normans, but Nicastro revolted against Robert Guiscard and his brother Roger. Having dominated the revolt, the new nobiliary had the castle built, which was restored in the 13th century by Frederick II of Swabia. It served as the place of imprisonment of Frederick II's son Henry.
The castle was destroyed by the disastrous earthquake of 1638 and lies today in ruins.
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.