The fortress of Trongzund (or Trångsund, lit. narrow strait) was built by the order of Peter the Great in the beginning of the 18th century after the Tsardom of Russia had captured the area from Sweden during the Great Northern War. In 1812, Trongzund was included by Alexander I into the newly created Grand Duchy of Finland.
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.