Niederkastenholz castle is surrounded by a moat, with an angular main castle and a separate horse-shoe shaped fore-castle with working quarters. It used to belong to Kornelimünster Abbey. The rough stone buildings of the castle are complemented by the wrought-iron gate. The fortified tower dates back to the 12th century and is the oldest part of the castle.
The so-called Probstei was added onto the medieval main castle between 1752 and 1756. Part of the moat is still visible today, although most of it was filled in during the 19th century.
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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.