The dominant feature of the Diez townscape is the high medieval castle. The oldest parts of the structure date from the eleventh century. The castle was abandoned as a residence from the mid-1700s, and from 1743 to 1784, the count's castle was used as a Nassau office building. Subsequently, it served as a prison until 1927.
During World War II, Diez Castle was commandeered by the German Army to interrogate 'prisoners of special interest' captured on the western front, one prisoner being Gertrude Legendre, the female Office of Strategic Services clerk/translator captured in 1944. In the eighteenth and nineteenth centuries, the castle was the site of the largest processing center of Lahn marble.
Since June 2006 it has served as a youth hostel and, since 2007, also contains a museum.
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.