[3] Along with Hämeenlinna, Olavinlinna in Savonlinna, Raseborg, and Turku, Kastelholm is one of only five surviving Finnish medieval fortresses that are also considered to be architecturally substantial.

When Niels Eriksen Gyldenstjerne, Danish Steward of the Realm during the period of 1453–1456, received the Kastelholm fief in 1485, he did so with the understanding that he was "faithfully to build and improve the walls and buildings of the said castle of Kastelholm, which are needful for the said castle, to the benefit and use of the crown of Sweden, as he has promised us willingly to do".

[12] Of the several enhancements made to the original construction, one of the most notable was by Gustav Vasa, before he became king of Sweden,[2] who regularly used to hunt on the castle grounds.

[17] John III of Sweden kept his deposed brother Eric XIV in captivity in the castle in the Fall of 1571.

At that time (1568-1621) the castle was the fief of queen dowager Catherine Stenbock, a political enemy of her stepson, Eric XIV.

[2] Built of brick and mortar,[21] the castle's original design included a rectangular stone keep and a residential wing.

[23] The castle is a major tourist attraction easily accessible by car from Mariehamn and by bus only on weekdays.

Blasting plans to build a new road some 700 metres (2,300 ft) from Kastelholm caused considerable controversy and dispute.

[25] In the end, experts assessed that the way the vibrations would be distributed would not cause the damage that was anticipated and the blasting went ahead.