Kastelholm Castle (Kastelholman linna, Swedish: Kastelholms slott) was built in the Middle Ages, in the late 14th century, and it is located in Sund, Åland. The castle played a major role in keeping Swedish rule for several centuries over the Baltic although the Danes were able to seize it sometimes.
The castle was built on a small island and was surrounded with several lines of poles. The castle, built of stone and brick, comprises a forecourt and a main castle in center of which is a high tower, “Kuuritorni”. The castle served as the administrative center of Åland and it held its position until Åland was annexed to the province of Turku in 1634. Several nobles and kings, e.g. King Gustav of Vaasa stayed there over the years.
Kastelholm Castle was damaged and rebuilt several times over time. As the importance diminished, it was also used as a prison. In a fire in 1745, it was badly damaged and began to decay. The northern wing survived the fire best, and served as a grainary until the 1930s. After that, the building began to be the first cultural history museum in Åland. The ruins of the castle were restored as early as the 1890s and the last renovation took place in 2001.
The castle is a popular tourist destination and is open to visitors from spring to autumn.