The Hohensyburg, a castle complex of the Lords of Sieberg, was constructed on the grounds of a former Saxon refuge, which was conquered in 775 by the Franks under Charlemagne. The castle, which was built around 1100 of Ruhr sandstone, was partially destroyed in 1287 by Count Eberhard I. von der Mark. The castle complex was an imperial fief of the von der Mark counts from 1300. This fiefdom was transferred to Brandenburg in 1609, and later to Prussia.
Two keeps, residential quarters (two-chamber system), the wall ring and the walls around the courtyard complex are still recognisable. In the inside of the castle is a war memorial by Fritz Bagdons.
References:Visby Cathedral (also known as St. Mary’s Church) is the only survived medieval church in Visby. It was originally built for German merchants and inaugurated in 1225. Around the year 1350 the church was enlarged and converted into a basilica. The two-storey magazine was also added then above the nave as a warehouse for merchants.
Following the Reformation, the church was transformed into a parish church for the town of Visby. All other churches were abandoned. Shortly after the Reformation, in 1572, Gotland was made into its own Diocese, and the church designated its cathedral.
There is not much left of the original interior. The font is made of local red marble in the 13th century. The pulpit was made in Lübeck in 1684. There are 400 graves under the church floor.