|Stegeborg Castle is a ruined castle in St Anna parish, Söderköping, Östergötland, located on an island in a narrow sound at the bay of Slätbaken.|
The oldest part of the castle is a square brick tower in the southeast corner, built in the early 1200s, when Söderköping became a city. A simple fortified residence was added during the 1300s, expanded and supplemented by a wall and a round tower in the west. During the 16th and 17th centuries the building was further expanded, especially during the reign of John III of Sweden, when the round tower attained its present height. In addition, a castle church was added to the north wing. Several of the Vasa kings lived in Stegeborg, which was allotted 1652–1689 to the Palatine counts John Casimir (died 1652) and Adolph John. The crown sold the castle in the 1730s for building materials. The bottom floor's large masonry protected it against complete destruction. Until 1689 the castle was the center of the Östergötland archipelago's local government. The new castle, a whitewashed three-storey building in neoclassical style, located at the former southern royal farm, was built in 1806.