During centuries, fortification walls and towers were built to protect ancient Nessebar. The first inhabitants on the peninsula were the Thracians who started to build these walls in the 8th century BC. From this period, a fortified wall and gate were found and can be seen today. After the decay of the Roman Empire, Byzantine rulers started an active construction work in the middle of the 5th century AD. The built walls were in style opus mixtum- an alternation of bricks and stone. Best preserved walls can be seen today near the city gate. The early Byzantine construction system was used in the middle ages as well.
Due to enemy war attacks, part of the walls were destroyed, but then quickly rebuilt. After the Ottoman rule in 1453, the fortification walls lost their significance and strategic importance and were left in ruins.
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.