The earliest sources of Gagi Castle dates from the 11th century. The Armenian historian Vardan (13th century) reported that it was built by King Gagic I (990-1020). The ruins of the castle however date from the older ages and probably Gagik I just rebuilt them.
The castle controlled the route leading to Tbilisi from the south. During the 15th century it also got known as Aghjakala, meaning 'White Castle', due to the color of its walls.
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.