Kastelholm Castle (), the only castle in Åland, is partially in ruins. The castle was built on a small island that was surrounded by water and moats filled with several lines of poles. It was first mentioned in 1388 in the contract of Queen Margaret I of Denmark, where a large portion of the inheritance of Bo Jonsson Grip was given to her. The heyday of the castle was in the 15th and 16th centuries. King John III of Sweden kept his deposed brother Eric XIV in captivity in the castle in the autumn of 1571. Today, partially restored, it has become a popular tourist attraction. The whole castle opened to the public in 2001.