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.
The site of Marstrand was chosen because of its location and its access to an ice free port. Initially a square stone tower was constructed, but by 1680 it was reconstructed and replaced by a round shaped tower. Successive additions to the fortress were carried out, by the inmates sentenced to hard labor, until 1860 when it was reported finished. Carlsten was exclusively a prison for men: Metta Fock in 1806-1809 was the only female prisoner to have been kept here. The most famous prisoner of Carlsten was arguably Lasse-Maja, who was imprisoned here in 1813-1839. The fortress was decommissioned as a permanent defense installation in 1882, but remained in military use until the early 1990s.