St Michael's Isle, popularly referred to as Fort Island, is noted for its attractive ruins. There is evidence for human activity on the island from the Mesolithic period onwards and there are two ancient buildings on the island. Both are in a state of ruin and closed to the public, though there are a number of walks which allow visitors to explore the surroundings.
St Michael's Chapel, a 12th-century chapel, is on the south side of the island. This Celtic-Norse chapel was built on the site of an older Celtic keeill.
The island is the site of two great battles for the control of the Isle of Man in 1250 and 1275, when England, Scotland and the Manx were fighting for control of the island. The Manx won the first battle, but 25 years later they lost control to Scotland.
Derby Fort, a 17th-century fort, is at the eastern end of the island. It was built by James Stanley, the 7th Earl of Derby and Lord of Mann in 1645, during the English Civil War, to protect the then busy port of Derbyhaven.
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.