Carlsten Fortress
A historic stone fortress overlooking the harbor of Marstrand on Sweden’s western coast.
Last updated August 25, 2026
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Overview
Carlsten Fortress, also known in Swedish as Karlstens fästning, is a historic fortification on the island of Marstrandsön in Marstrand, on Sweden’s western coast. The fortress was established after the Treaty of Roskilde in 1658 transferred Bohuslän, including Marstrand, from Denmark-Norway to Sweden. Because Marstrand had an important trading harbor that was largely free of ice during winter, the settlement was strategically valuable and suitable as a naval base. King Carl X of Sweden ordered the construction of defenses to protect the newly acquired province and the harbor from attacks. The first defensive work was a temporary wooden fortification known as Wahlen skans, constructed in 1658 under the direction of Johan Wärnschöld on the mountain above the town. It helped resist a Norwegian attack in the autumn of 1659. A more permanent replacement began in 1666 in the form of a stone redoubt with associated ravelins. A square tower followed, and between 1671 and 1673 the structure was raised by another level and equipped with a roof battery. The developing fortress took the name Karlsten or Carlsten. The complex was substantially redesigned and enlarged during the late seventeenth century. In 1681, work began on a large circular tower associated with the plans of commander Carl Gustaf Frölich. Erik Dahlbergh’s designs were approved in 1685, and by 1689 the principal fortress was close to its later form, combining a brick redoubt with a tall, multi-storey tower that incorporated the earlier square structure. Construction and later expansion continued for many years, with additions and improvements extending until approximately 1860. Much of the labor was performed by prisoners sentenced to penal labor. Carlsten was tested in two major wartime episodes. On 23 July 1677, during the Scanian War, Danish forces under Ulrik Frederik Gyldenløve attacked and captured the Marstrand defenses. In 1719, during the Great Northern War, Norwegian forces led by Vice Admiral Peter Wessel Tordenskjold besieged the fortress and again forced its surrender. Carlsten was subsequently returned to Swedish control through diplomatic settlements and treaties. The fortress also served as a prison, exclusively for male prisoners according to the historical account, although the female prisoner Metta Fock was reportedly held there from 1806 to 1809. One of its best-known prisoners was Lasse-Maja, a celebrated Swedish thief and cross-dresser who was incarcerated at Carlsten from 1813 to 1838. The site was decommissioned as a permanent defense installation in 1882, but it continued to have military uses until the early 1990s. Today, Carlsten is primarily a heritage landmark and visitor attraction rather than an operating military installation. Its identity is built around its imposing architecture, views over the Kattegat and Marstrand harbor, military history, prison history, and role in the development of Sweden’s western defenses. The fortress is associated with the broader cultural and tourism offering of Marstrand, although the available reference material does not identify a specific current owner, operating company, or formal corporate parent.
History
Carlsten Fortress emerged from Sweden’s strategic need to defend Bohuslän after the Treaty of Roskilde in 1658. The treaty brought the province and the harbor town of Marstrand under Swedish control. Marstrand’s position on the western coast, together with its commercially important and relatively ice-free harbor, made it a useful naval and defensive base. King Carl X Gustav therefore ordered the creation of a permanent fortification on the mountain above the town. The first work was a temporary wooden position called Wahlen skans. Built in 1658 under Johan Wärnschöld, it was used against a Norwegian attack in 1659. Its replacement began in 1666, when a stone fortification in the form of a Roman-style redoubt was started with four small ravelins. A square tower was then built, and between 1671 and 1673 the tower was raised and fitted with a roof battery. These successive stages established the core of what became known as Karlsten or Carlsten. A major rebuilding phase began in 1681 with construction of a large circular tower. The work was connected with the design of commander Carl Gustaf Frölich, while broader plans associated with Erik Dahlbergh received approval in 1685. By 1689, the main fortress was nearly complete in the form that defined its later appearance: a brick redoubt combined with a high, multi-level tower incorporating the earlier square structure. Expansion, reinforcement, and completion work continued intermittently until about 1860. Prisoners sentenced to penal labor supplied much of the construction workforce, adding a penal dimension to the site’s history. Carlsten was captured twice during periods of conflict between Sweden and its neighboring powers. On 23 July 1677, Danish forces commanded by Ulrik Frederik Gyldenløve attacked and took the fortress during the Scanian War. In 1719, Norwegian forces under Vice Admiral Tordenskjold mounted a second siege and captured the position. The fortress later returned to Swedish control through negotiations and treaties rather than an immediate reconquest described in the available material. Beyond its military role, Carlsten was used as a prison. It was intended primarily for male prisoners, although Metta Fock, a female prisoner, is reported to have been held there between 1806 and 1809. Lasse-Maja, one of Sweden’s most famous historical criminals, was imprisoned at Carlsten from 1813 to 1838 and remains closely associated with the fortress’s public history. Carlsten ceased to function as a permanent defense installation in 1882. Nevertheless, the Swedish military continued to use the site in some capacity until the early 1990s. The fortress has since been understood mainly as a preserved historic monument and visitor attraction. Its current public identity combines architecture, military history, maritime strategy, prison history, and the wider tourism profile of Marstrand. The supplied references do not establish a current legal owner, a corporate operating structure, or a named executive team.
- 1990
Military use ends in the early 1990s
[1]The fortress’s remaining military use is reported to continue until the early 1990s, after which its public identity is primarily historical and touristic.
- 1882
Permanent defense role ends
[1]Carlsten is decommissioned as a permanent defense installation, although military use continues in some form for many more years.
- 1860
Long construction program reaches completion
[1]Successive additions and improvements to the fortress are completed after a construction history extending over roughly two centuries.
- 1719
Norwegian siege and capture
[1]Norwegian forces under Vice Admiral Tordenskjold besiege and capture Carlsten for a second time in its history.
- 1689
Main fortress approaches completion
[1]The principal complex, including the brick redoubt and tall tower, is described as nearly complete.
- 1685
Dahlbergh’s broader plans receive approval
[1]General plans associated with Erik Dahlbergh are approved as the fortress is developed into a larger and more complex defensive work.
- 1681
Large circular tower is started
[1]A major rebuilding program begins with construction of the large round tower that became central to the fortress’s later appearance.
- 1677
Danish capture of Carlsten
[1]On 23 July, Danish forces led by Ulrik Frederik Gyldenløve capture the fortress after attacking the Marstrand defenses.
- 1671
Tower construction advances
[1]The developing tower is built up by an additional level and receives a roof battery during the 1671–1673 construction phase.
- 1666
Permanent stone replacement begins
[1]Construction starts on a more substantial stone fortification intended to replace the temporary wooden defenses.
Products and positioning
A major Swedish military-heritage landmark positioned as a historic fortress and cultural tourism destination in Marstrand.
Heritage attraction
Carlsten Fortress visitor experience
The central offering is access to the preserved fortress complex on Marstrandsön. Visitors encounter the tower, redoubt, defensive structures, elevated views, and the physical setting that shaped the site’s military importance. The experience is based on the fortress as an architectural and historical landmark rather than on a conventional consumer product.
Cultural tourism
Historical tours and interpretation
Tours and interpretation can present Carlsten’s development after 1658, its role in protecting Bohuslän and Marstrand, the Danish and Norwegian captures, the long construction program, and the fortress’s use as a prison. The supplied material identifies the historical basis for these experiences but does not specify a particular tour operator or current program schedule.
Brand decisions
1882
End of permanent defense statusStrategyBackground
Changing military requirements reduced the need for Carlsten to operate as a permanent defense installation.
Action taken
The fortress was decommissioned from permanent defense service.
Aftermath
Military use continued until the early 1990s, after which the site’s principal public role became that of a historic landmark and visitor attraction.
1681
Expansion into a major fortressGeneration changeBackground
Sweden continued to strengthen the position after earlier construction phases and the experience of regional conflict.
Action taken
A large circular tower was begun, followed by broader plans and additional construction associated with Carl Gustaf Frölich and Erik Dahlbergh.
Aftermath
The resulting complex became one of the most prominent fortifications on Sweden’s western coast.
1666
Replacement of the temporary fortificationGeneration changeBackground
The first wooden defense was considered inadequate as a lasting protection for Marstrand and the newly Swedish province.
Action taken
Construction began on a stone redoubt and associated defensive works.
Aftermath
The stone structure became the foundation for later tower and fortress expansions.
1658
Establishment of a Swedish coastal defenseStrategyBackground
After Sweden acquired Bohuslän and Marstrand through the Treaty of Roskilde, the harbor’s strategic and commercial importance made local defenses a priority.
Action taken
King Carl X ordered a fortification on the mountain above Marstrand, beginning with the temporary Wahlen skans.
Aftermath
The initial position evolved through repeated rebuilding into the permanent Carlsten fortress complex.
Sources2 sources
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