The ancient castle Castra Martis was built on a steep southern slope above the gorge of the Voynishka River at the end of the 3rd-beginning of the 4th century as part of the Danube Limes. It occupied a key position in guarding the important Roman road from Bononia to Singidunum (Belgrade).
In 408, the Huns under Uldin took control of the site during an attack on the Eastern Roman Empire, apparently by treachery. The fortification was reinforced under Justinian I, but was completely abandoned after the Gothic invasions in 586.
The fortress consisted of two parts: a small square fortification (quadriburgium) measuring 40x40 m, with powerful circular towers with a diameter of 12.5 m in the corners and a castle with the shape of an irregular quadrangle. It covered an area of 1.55 ha. The castle was accessible only from the south, where the gate was situated. Probably at the end of the 4th century, the protection of the gate was reinforced by the construction of another narrower wall.
Foundations of a Roman bath were discovered northwest.The quadriburgium is well-preserved and revealed in its entirety. Its fortress walls, built of stone and three-row brick girders, are 2.2 m thick and 2 m tall, and the southeast tower rises to 16.3 m.
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.