Carlsten is a stone fortress 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 labour, 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.
The fortress was attacked and sieged twice falling into enemy hands. In 1677 it was conquered by Ulrik Frederick Gyldenløve, the Danish military commander in Norway and in 1719 by the Norwegian Vice-Admiral Tordenskjold. At both occasions the fortress was returned to Swedish control through negotiations and treaties.
References:The Broch of Clickimin is a large and well preserved, though somewhat restored broch near Lerwick. Originally built on an island in Clickimin Loch (now increased in size by silting and drainage), it was approached by a stone causeway. The water-level in the loch was reduced in 1874, leaving the broch high and dry. The broch is situated within a walled enclosure and, unusually for brochs, features a large 'blockhouse' between the opening in the enclosure and the broch itself. Another unusual feature is a stone slab featuring sculptured footprints, located in the causeway which approached the site. Situated across the loch is the Clickimin Leisure Centre.