The Sponeck castle was built in the late 13th century to the site of late Roman fort (built by Emperor Valentinian I around 365 AD). The castle was destroyed in the Thirty Years War. After several changes of ownership the painter Hans Adolf Buhler acquired it in 1917, rebuilt and set up a studio in the former residential tower. After the painter's death in 1951 the castle was owned by the family. Today the stone wall and two corner towers are preserved of an original Roman fort.
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.