During the 13th century Prussian Crusade, the current Gvardeysk area was conquered by the Teutonic Knights. To protect Samland from the Nadrovians and Scalovians, the crusaders built a wooden fort between the Deime and Pregel rivers from 1283–90. This was replaced by Tapiau Castle, a stone Ordensburg, in 1351. Vytautas, forthcoming grand duke of Lithuania was baptised here in 1383 after he breached of Krėvė prison. Since 1450 town and castle was named Tapiau. After Teutonic state was downed by Poland and Lithuania, Teutons moved order archives to Tapiau castle and kept it during 1469-1722. Later Tapiau castle was abbandoned. Soviets built a jail on the mound, it remains active.
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.