San Nicandro castle as we see it today dates from the 15th century although it was built on the site of a much more older fortress dating from the Norman and Swabian periods.
The castle is trapeze shaped with four corner towers, and was renovated during the 17th century. On the north side there are two square towers, one of which is one of the original towers, while on the south side there are two round towers built by the Aragonese which are behind the church.
The castle, which was actually built as a mansion, was the usual residence of the Della Marra and all the vassals of San Nicandro Garganico until the time of the Cattaneo Princes. In the 16th century the castle was integrated with the gateway to the east and the lodge was built that connected the castle with a large palace built between the door and the walls of the castle.
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.