Built in a strategic location to protect the Sund Strait, Kastelholms Castle played a crucial role in defending the country. The fortress' north wing retraces the history of the site and the neighbouring open-air museum, known as Jan Karlsgården, gives an idea of the lifestyle of a rich peasant living on the island in the 18C and 19C. Festivities such as those on Midsummer's Day enliven the summer months. The small white building adjoining the entrance to the museum was the only prison of the Åland Islands for 200 years. It houses the modest Vita Björns Prison Museum, which illustrates prison life from 1800 to 1950.