Kastelholm Castle is a Swedish-built medieval castle located off Road 2 in Sund, Åland, Finland, approximately 25 km (16 mi) northeast of Mariehamn, overlooking a fjord to the south of the village of Kastelholm. Along with Hämeenlinna, Olavinlinna in Savonlinna, Raasepori, and Turku, Kastelholm is one of only five surviving Finnish medieval fortresses that are also considered to be architecturally substantial. Built in the 14th century, and held in fief during the Middle Ages by various nobles, feudal chiefs, and kings, it had significant period in the 15th and 16th centuries.
Built in the 14th century, originally on a small island surrounded by moats filled with water and planted with several rows of poles, the castle has been of strategic importance in consolidating Swedish authority over the Baltic over the last several centuries; with several Swedish monarchs parading through the history. It was first damaged in the 1599 civil war when it came under king Charles IX control and was rebuilt by 1631. The castle was gutted and ruined in 1745. In the 1930s, it was partially used as a granary. However, since then it has been refurbished and is now an important part of the tourist circuit in Åland.