Carlsten Fortress (Swedish: Carlstens fästning) is a fortress in the city of Marstrand, located on the west coast of Sweden.
It was built in 1658-1659 by orders of King Charles X Gustav of Sweden to protect the country's western coast against hostile attacks. The location was chosen because of its proximity to a port that rarely freezes over, allowing a part of the Swedish navy to be stationed there. It had been in military use until the early 1990s.
The fortress is now a museum open to the public, hosting tours and Viking or medieval-themed dinners with recipes dating back 300 years and offering a unique experience that takes everyone back to the old days.