The construction of Gerione Castle was probably motivated by the need to control mountain routes and its strategic position as an observation point for a large part of the Sele plain. Documented for the first time in a parchment from 1056 during the Lombard period, the exact date of its construction is unknown.
With the Assise of Capua in 1220, the Gerione Castle was acquired by Frederick II of Sweden for the Demanio Regio, becoming directly dependent on the emperor and included among the Castra exempta, the main fortifications of the kingdom.
The structure maintained its military-defensive function until 1515 when, due to its inconvenient location for the needs of the time, it was handed over to the Collegiate Church of Santa Maria della Pace by the last feudal lord to possess it, Duke Ferdinando Orsini. According to others, the castle was used both as a prison and as a Spanish garrison during the feudal rule of the Grimaldi Princes of Monaco, Marquises of Campagna.
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.