Carlsten (Swedish: Karlstens fästning) 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.