Olbrück Castle is a hilltop fortress with a diamond-shaped floor plan. The central feature of the castle is the 34m high main tower, which dates back to the mid-14th century and is the oldest preserved part of the structure. Today, these rooms serve as museum spaces where visitors can learn about the construction history and function of castles. From the tower's roof, visitors have a panoramic view of the Rhine River to the east, the Ahr Mountains to the north, and the volcanic landscape to the south. On clear days, the Cologne Cathedral is even visible.
The castle was probably built originally in the 12th century. After several owners and reconstructions in the Late Middle Ages, During the Thirty Years' War, the castle was captured and devastated by Swedish troops in 1632. It was recaptured by Spanish and Electorate of Cologne troops under Graf Ernst von Isenburg-Grenzau in January 1633. Partially collapsed in 1660, the castle was subsequently restored under the supervision of Capuchin friar Bonitius from Linz.
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.