The site of current Seeburg castle was mentioned first time in 740 AD. The construction of current castle was started in the 11th century (around 1036). It was largely extended by Wichmann von Seeburg, later Archbishop of Magdeburg (1115-1192). The next renovations took place in the 14th and 15th centuries, when the castle was flanked by towers and a gatehouse under the rule of Counts of Mansfeld.
Later Seeburg was left to decay until the Counts of Ingenheim sold it in 1880 to the Wendenburg family. In 1923, Erich Wendenburg commissioned the architect Paul Schultze-Naumburg to completely restore the building in neo-Gothic style.
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.