Kastelholm Castle (Swedish: Kastelholms slott, Finnish: Kastelholman linna) is a medieval castle built by the Swedes in the late 14th century. It is located near Road 2 in the municipality of Sund in Åland, Finland, about 25 kilometres (16 mi) northeast of Mariehamn, overlooking a fjord near the village of Kastelholm. . Kastelholm is one of five surviving medieval castles in Finland considered architecturally significant, along with Hämeenlinna, Olavinlinna, Raseborg, and Turku.
https://en.wikipedia.org/wiki/Kastelholm_Castle