Bohus Fortress Bohus Fortress ( also " Baahus " or " Båhus " , originally : " Bagahus " ) lies along the old Norwegian - Swedish border in Kungälv , Bohuslän , Sweden , north east from Hisingen where the Göta river splits into two branches ( 20 kilometres north of Gothenburg ) . It commands its surroundings from a cliff 40 metres high , with the river forming a natural moat around it . Initial construction . Bohus Fortress ( , ) construction began in 1308 under King Haakon V Magnuson , king of Norway from 1299 until 1319 . Håkon V also initiated construction of Norwegian fortresses at Akershus and Vardøhus as part of a broader defensive policy . At the time Bohuslän ( " Båhuslen " ) was Norwegian territory and it served as a main Norwegian defence against Sweden along the coast as well as the strong point for the Bohuslän region from 1308 until 1658 . Medieval castle . According to architect Guthorm Kavli , “ by 1310 records show it was constructed , as normal for that period , out of granite and brick , perhaps under the guidance of Count Jacob of Halland . By 1450 it included a continuous surrounding wall , 3 metres thick at the base , with a height which varied from 8 . 5 to 13 . 5 metres , varying with the terrain . It was approximately rectangular , with four rectangular corner towers . At the eastern end there was a brick tower , and in the centre of the west side a gate house and drawbridge . Along the inside of the surrounding wall buildings were located which among other things included the " Kings hall , " the castle commander ’ s residence , the chapel , the guardroom , the barracks and the kitchen . The fortress had secure vaulted positions , partly cut into the mountain , and beyond that strong outer - works . At the time Båhus was Norway ' s strongest fortress . The approaches were very difficult and the area to be defended was small , only 250 x 150 metres , so it did not require a large defensive force . ” Fortress . The fortress was invested numerous times , but was never captured . During the Northern Seven Years ' War ( 1563 – 1570 ) it was seriously damaged . This occurred in 1566 , when 250 Swedish soldiers successfully stormed the northeastern - most tower . The Norwegian commander sent a volunteer to blow up the ammunition stores underneath the tower , killing the Swedes and repelling the attack . As a reward the family of the volunteer got a piece of land which is still in property of the descendants of this volunteer . The Norwegians rebuilt the fortress of stone and brick , and substantially reinforced it . The reconstruction immediately after the war was directed by Hans Paaske ( Påske ) from the Netherlands . In 1593 - 1604 , similar to the construction then undertaken at Akershus in Oslo , Bohus was upgraded to a bastion fortress . A new outer fortification was raised . This construction was one of the early works by Hans van Steenwinckel , also from the Netherlands , who was later famous for his Dutch Renaissance style design in Denmark . As Swedish invasions continuously threatened Norwegian Båhuslen in this period , the improvements to the fortifications continued for years . For example from the summer of 1651 through the autumn of 1652 the Dutch engineer Isaac van Geelkerck directed the construction of two corner towers along the south face and a new ring wall was constructed around the arsenal building . Loss to Sweden . Under the terms of the Treaty of Roskilde in 1658 , Denmark – Norway ceded the Danish provinces of Scania , Blekinge and Halland ( the latter was agreed to be Swedish for a period of 30 years after the Peace of Brömsebro , but was in the treaty of Roskilde given to Sweden permanently ) and the Norwegian provinces Trondhjem and Bohuslän ( including Bohus Fortress ) . After Denmark – Norway ceded the territory which included Bohus Fortress , Fredriksten Fortress was constructed in Fredrikshald on the newly established Norwegian - Swedish border . Since Bohus Fortress no longer lay on the border , it was of minimal future use to Sweden , which relied on the existing Älvsborg Fortress at Gothenburg and a new Carlsten Fortress erected at Marstrand . As prison . Instead the fortress was used as a prison . The most famous prisoner was the radical pietist Thomas Leopold , who during his life spent 42 years behind bars , 32 years at Bohus , for his alleged heresies . His stone cell can be visited at the castle today . Modern times . At the end of the 18th century it was decided that the now unused fortress should be demolished . Demolition crews worked at the fortress for two months , at which time the money allocated for the task had run out . Residents of the surrounding town of Kungälv used the dressed stone of the fortress for building houses . However , much of the fortress is still intact , including the large northern tower , " Fars hatt " . The fortress is now a museum and open to tours in the summer .