Tunaborgen is a ruined former bishop castle. In 1291 Archbishop Nils Alleson mentioned a fortification on the site. The fort was also a strategic point in the Gustav Vasa's war against the Danes.
The ruins were rediscovered around 1920. The castle consisted of a square tower, a citadel, built together with an almost square walls and it was surrounded by a circular moat. Today the ground floor remains.
References:Château de Niort is a medieval castle in the French town of Niort. It consists of two square towers, linked by a 15th-century building and dominates the Sèvre Niortaise valley.
The two donjons are the only remaining part of the castle. The castle was started by Henry II Plantagenet in the 12th century and completed by Richard the Lionheart. It was defended by a rectangular curtain wall and was damaged during the Wars of Religion. In the 18th century, the castle served as a prison.
The present keeps were the central point of a massive fortress. The southern keep is 28m tall, reinforced with turrets. The northern tower is slightly shorter at 23m. Both are flanked with circular turrets at the corners as well as semicircular buttresses. Each of the towers has a spiral staircase serving the upper floors. The Romanesque architecture is of a high quality with the dressed stones closely jointed.