During the Middle Ages, an Old Prussian fort called Malcekuke was located near the current site of Pieniężno. The Teutonic Knights built an Ordensburg castle near Malcekuke in 1302. Both the castle and the town which developed nearby were destroyed during the war between the Teutonic Order and the Kingdom of Poland in 1414. During the Thirteen Years' War, Mehlsack surrendered to the Order, and the castle burned down during Poland's recapture of the town. From 1589-1599, Prince Andrew Cardinal Báthory of Transylvania, cousin of Sigismund Báthory, was the administrator for the castle. In 1550, the Prussian army laid siege to the city and partially burned it down.
The town was captured by Swedish troops in 1626 during the Polish-Swedish War of 1625-29, recovered by Hetman Stanisław Rewera Potocki, and then had its castle partially destroyed by Swedish troops in 1627. The castle was restored in 1640 with Baroque gables, and its function changed from being a fortress to being a château.
During the 19th and 20th centuries the castle lost some of its Gothic and Baroque features, and in 1870 its eastern and southern wings were demolished after extensive deterioration. The remainder of the castle was used as administrative offices for Prussianofficials. From 1920-31 the western wing was renovated so the castle could be used as a school and museum. In 1945, Mehlsack, including its castle, was 90% destroyed by fighting during World War II and was conquered by the Soviet Red Army from Nazi Germany.References:
First record of Kastelholma (or Kastelholm) castle is from the year 1388 in the contract of Queen Margaret I of Denmark, where a large portion of the inheritance of Bo Jonsson Grip was given to the queen. The heyday of the castle was in the 15th and 16th centuries when it was administrated by Danish and Swedish kings and stewards of the realms. Kastelhoma was expanded and enhanced several times.
In the end of 16th century castle was owned by the previous queen Catherine Jagellon (Stenbock), an enemy of the King of Sweden Eric XIV. King Eric conquered Kastelholma in 1599 and all defending officers were taken to Turku and executed. The castle was damaged under the siege and it took 30 years to renovate it.
In 1634 Åland was joined with the County of Åbo and Björneborg and Kastelholma lost its administrative status.