Carlstens Fästning

Sweden, Sweden

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Carlstens Fästning is a prominent stone fortress perched on the highest point of Marstrand Island on Sweden's west coast, built starting in 1658 on orders from King Carl X Gustav after the Treaty of Roskilde to safeguard the newly acquired Bohuslän province and its ice-free harbor.

Evolving from a wooden redoubt under Johan Wärnschöld into a robust structure with a massive round tower designed by Erik Dahlbergh by 1689, it endured sieges in 1677 by Danish forces under Gyldenløve and in 1719 by Norwegian admiral Tordenskjold, both times regained through treaties, while serving as a prison using convict labor. 

Decommissioned as a defense site in 1882 yet used militarily until the 1990s, it now operates as a museum, event venue, and hotel offering concerts and overnight stays amid stunning coastal views.