Commander of the sea: Östanå Castle, Sweden

For centuries, Östanå Castle, a hill castle, controlled the main shipping channel into Stockholm through the Stockholm archipelago. Various families, hailing from the Swedish nobility, claimed ownership of the castle between the 14th and 17th centuries. Unfortunately, it was destroyed during the Russian Pillage in 1719 and rebuilt between 1791-1794. In the 19th century, Erik Gustaf Boström, Prime Minister of Sweden, resided in the castle. The Boström family relinquished ownership in 2006, and by 2016, the castle was in need of repair and was listed for sale at 35 million kronor.

Sources: Wikipedia, OpenStreetMap, Wikidata, Wikimedia Commons