In 1264, castrum Hackenberg is mentioned as a site for document sealing. After the last Hackenberger died, the property passed to Alber Stuchs von Trautmannsdorf in 1382. Christoph Kuenritz bought and converted it into a Renaissance castle in 1543. Plundered in 1620 during the Thirty Years' War, it remained devastated until Sigmund Friedrich von Sinzendorf purchased it in 1650. His nephew, Theodor Reichsgraf von Sinzendorf, remodeled it into a Baroque palace between 1679 and 1677. After Prosper von Sinzendorf's death in 1822, it passed to the Reuß-Köstritz princes but fell into disrepair.
In 1945, it came under Soviet administration as German property, and in 1955, it was returned to Prince Reuß. Artists from the Vienna Group, including Friedensreich Hundertwasser, moved in from 1959. Captain Josef Steiger bought it in 1974, and in 1986, Horst Wächter, Hundertwasser's former assistant, acquired it. The Osmann family took over in 2020, with Horst Wächter remaining for historical research.
The four-story structure features Renaissance geometry and proportions, with medieval foundations. A cosmological mysticism, likely embedded during the 1679 Baroque renovation, is present. The four wings align precisely with the cardinal directions, enclosing a rectangular courtyard. The southern wings retain colorful ceiling frescoes and stucco decorations.
Access is via a short alley in the northeast, crossing a stone bridge into a frescoed entrance hall, leading through the courtyard with an octagonal fountain to a three-bay shell grotto, the 'sala terrena,' decorated with statues, frescoes, shells, and stucco, extending into the garden. The former access extended over the moat to a landscaped area with a Baroque-era artificial lake.
The Initiative Hagenberg association hosts cultural events at the castle.
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.