Maasi medieval fort-castle was built with the forced labour of islanders. That's how the ruling Liivi order punished indigenous inhabitants for the uprising, which had destroyed orders previous stronghold. Seaside fort-castle was undefeated until destroyed by Danes. The fortress was blown up in 1576 by the Danes in an attempt to forestall the Swedish invasion and nothing was done for the next 300 years. 8m walls that survived the destruction have become a landmark.
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.