Stegeborg Castle, Medieval castle ruins in Skällvik, Sweden

Stegeborg Castle is a ruined castle on an islet in the narrow sound at the bay of Slätbaken. Its oldest part is a square brick tower in the southeast corner, built in the early 13th century; a walled residence with a round tower in the west was added in the 14th century. The round tower reached its present height under John III, and a castle church was added to the north wing. The lower storey's thick masonry survived the 1730s sale of the castle for building materials.

The oldest part is a square brick tower from the early 13th century, when Söderköping became a city. A fortified residence followed in the 14th century, with a wall and a round tower in the west. John III raised the round tower to its present height in the 16th century and added a castle church to the north wing. The crown sold the castle for building materials in the 1730s, but the thick ground-floor masonry survived.

The name comes from "stæk," referring to wooden poles that were placed in water to manage ship traffic through the narrow sound. Today, visitors can still sense how this location controlled the movement of vessels through an important maritime passage.

The ruins lie on a small islet in the narrow sound at Slätbaken, reached from the mainland at St Anna parish near Söderköping. Land elevation since the 14th century has been about two metres, so the channel is shallow and there is no tidal range to plan around. Approach from the south side, the route vessels used because pilings blocked the northern passage.

The name Stegeborg comes from stæk, medieval Swedish for a stock or pile: wooden stake barriers sunk in the channel to block ships. The oldest date to the 7th–9th centuries, and 11th-century pilings were found in the water north of the island. They survived into the 19th century, forcing vessels to pass on the south side.