Carlsten is a stone fortress located at Marstrand, on the western coast of Sweden. The fortress was built on the orders of King Carl X of Sweden following the Treaty of Roskilde, 1658 to protect the newly acquired province of Bohuslän from hostile attacks, choosing this site because of its location and its access to an ice free port. Successive additions to the fortress were carried out, by the inmates sentenced to hard labor, until 1860 when it was reported finished. The fortress was decommissioned as a permanent defense installation in 1882, but remained in military use until the early 1990s. It was attacked and sieged twice falling into enemy hands, in 1677 and in 1719, being returned to Swedish control through negotiations and treaties. Carlsten Fortress is now used for military courses during the winter months and in the summer it is open to tourists.
