The castle has a long and varied history. Construction on what was originally a fortress, the only medieval fortress on Åland, began in the late 1300s. When exactly construction began is not known, but the castle is first mentioned in historical sources in 1388. Although the castle was originally located on a small islet, there is now only water on two sides of the building. At the time of the castle’s construction, Åland was an independent administrative district with Kastelholm as its centre.
Many dramatic events have taken place here, from sieges to fires. Due to its strategic location, the castle has taken part in many a political conflict, for instance the Engelbrekt rebellion in 1434. On another occasion, in 1507, the much-feared privateer captain Sören Norby and his fleet took the castle by surprise and seized it.
Kastelholm Castle became the seat of the Swedish king’s fief-holders and bailiffs. During the reign of Gustav Vasa, the castle turned into a royal hunting lodge. The king, who took a personal interest in Åland, had the castle and royal demesne close by expanded. In the early spring of 1556, Gustav Vasa and his family spent a couple of months at the castle.
That same year, Vasa’s son John was appointed Duke of Finland. Apart from staying at the castle in 1557, 1559 and 1561, Duke John kept his brother Erik XIV and his wife Karin Månsdotter imprisoned here during the autumn of 1571. A small room on the third floor of the Kure tower has traditionally been claimed to be where King Erik was held prisoner. All in all, the 16th century proved to be most eventful in the castle’s history.
Kastelholm continued to be Åland’s administrative centre up until 1634. During this time the castle was rebuilt and extended many times. In 1616 and 1622 the Swedish king Gustav II Adolf visited the castle. Following a fire in 1619, the very last royal governor on Åland, Stellan Otto Mörner, attempted to rebuild the castle as a more representative building. However, as time went by the castle lost its significance.
In 1745 yet another fire almost completely destroyed the castle which was then left to slowly fall into ruin. The north wing, however, survived the fire and came to be used as a granary. Since the late 1800s the castle has undergone several extensive restoration projects, the most recent taking place in the 1990s. Today the castle is a museum.