Hachen Castle, built around 1000, initially owned by Count Ezzo, later passed to Otto von Northeim in 1060. In 1080, Otto's son gifted it to the Archbishop of Cologne, who granted it as a fief to the Counts of Dassel. In 1232, the Counts of Dassel sold it to the Counts of Arnsberg. After changing hands multiple times, it became property of the municipality of Hachen in 1839. The castle's significance waned from the 15th century onwards. Limited research on its architectural history includes findings from 1923 and 1978–1980, revealing a main castle area, bailey, shield wall, and various structures.
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.