Lembecksburg was a medieval ring wall with a diameter of 95 meters and a height of ten meters. According to old lore, it was constructed in the 9th century as a stronghold against the Vikings and is named after the knight Klaus Lembeck who had allegedly been residing there as a steward of king Valdemar IV of Denmark in the 14th century. After breaking his feudal oath, though, Lembeck is said to have been besieged by the king's host. Ít is disputed though whether Lembeck ever set foot on the island. Archaeological findings on Sylt island in the late 1970s suggest, however, that the Lembecksburg and similar facilities on Sylt date back to the days of the Roman Empire.
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.