Hachen Castle, built around 1000, initially owned by Count Ezzo, later passed to Otto von Northeim in 1060. In 1080, Otto's son gifted it to the Archbishop of Cologne, who granted it as a fief to the Counts of Dassel. In 1232, the Counts of Dassel sold it to the Counts of Arnsberg. After changing hands multiple times, it became property of the municipality of Hachen in 1839. The castle's significance waned from the 15th century onwards. Limited research on its architectural history includes findings from 1923 and 1978–1980, revealing a main castle area, bailey, shield wall, and various structures.
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.