Bohus Fortress construction began in 1308 under King Haakon V Magnuson, king of Norway from 1299 until 1319. At the time Bohuslän 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.
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.
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.
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. 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.
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.References:
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