The dominant feature of the townscape is the high medieval Castle of Diez. 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.