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:
From its origin as a small stronghold built by the ancient Illyrian tribe Dalmatae, becoming a royal castle that was the seat of many Croatian kings, to its final development as a large fortress during the Ottoman wars in Europe, Klis Fortress has guarded the frontier, being lost and re-conquered several times. Due to its location on a pass that separates the mountains Mosor and Kozjak, the fortress served as a major source of defense in Dalmatia, especially against the Ottoman advance, and has been a key crossroad between the Mediterranean belt and the Balkan rear.
Since Duke Mislav of the Duchy of Croatia made Klis Fortress the seat of his throne in the middle of the 9th century, the fortress served as the seat of many Croatia"s rulers. The reign of his successor, Duke Trpimir I, the founder of the Croatian royal House of Trpimirović, is significant for spreading Christianity in the Duchy of Croatia. He largely expanded the Klis Fortress, and in Rižinice, in the valley under the fortress, he built a church and the first Benedictine monastery in Croatia. During the reign of the first Croatian king, Tomislav, Klis and Biograd na Moru were his chief residences.
In March 1242 at Klis Fortress, Tatars who were a constituent segment of the Mongol army under the leadership of Kadan suffered a major defeat while in pursuit of the Hungarian army led by King Béla IV. After their defeat by Croatian forces, the Mongols retreated, and Béla IV rewarded many Croatian towns and nobles with 'substantial riches'. During the Late Middle Ages, the fortress was governed by Croatian nobility, amongst whom Paul I Šubić of Bribir was the most significant. During his reign, the House of Šubić controlled most of modern-day Croatia and Bosnia. Excluding the brief possession by the forces of Bosnian King, Tvrtko I, the fortress remained in Hungaro-Croatian hands for the next several hundred years, until the 16th century.
Klis Fortress is probably best known for its defense against the Ottoman invasion of Europe in the early 16th century. Croatian captain Petar Kružić led the defense of the fortress against a Turkish invasion and siege that lasted for more than two and a half decades. During this defense, as Kružić and his soldiers fought without allies against the Turks, the military faction of Uskoks was formed, which later became famous as an elite Croatian militant sect. Ultimately, the defenders were defeated and the fortress was occupied by the Ottomans in 1537. After more than a century under Ottoman rule, in 1669, Klis Fortress was besieged and seized by the Republic of Venice, thus moving the border between Christian and Muslim Europe further east and helping to contribute to the decline of the Ottoman Empire. The Venetians restored and enlarged the fortress, but it was taken by the Austrians after Napoleon extinguished the republic itself in 1797. Today, Klis Fortress contains a museum where visitors to this historic military structure can see an array of arms, armor, and traditional uniforms.