[1] The oldest part of the castle is a square brick tower in the southeast corner, built in the early 13th century, when Söderköping became a city.

A simple fortified residence was added during the 14th century, 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.

The new palace, a whitewashed three-storey building in neoclassical style, located at the former southern royal farm, was built in 1806.

Its strategic location at the entrance to Söderköping, at the time one of the country's wealthiest cities, was obvious to Birger Magnusson's guardianship, which was led by the marsk Torkel Knutsson.

The edifice was probably not too extensive, as the island is currently not large and post-glacial isostatic rebound in Östergötland has resulted in land elevation of two meters since the 14th century.

He was able to receive Skällsvik and the castle ruins as a gift from Duke Erik's widow Ingeborg of Norway on January 27, 1321.

On February 28, 1350 the bishop of Linköping Peter Tyrgilsson signed a letter at the manor (in manerio nostro Skældowijk).

During the dynastic conflicts between King Magnus and Erik Magnusson in the 1350s, Skällvik manor was held by bishop Nils Markusson.

In 1391 Stegeborg was owned by the Crown, and Margaret had appointed the Danish knight Evert Moltke as sheriff.

In July it was agreed that Nils would voluntarily yield the castle in September unless King Eric sent some relief.

In late August, Eric returned to Gotland and in the autumn Stegeborg was handed to the Privy Council.

Erengisle seems to have been friendly to the Kalmar Union but was fired as sheriff in 1463 when the king suspected him of sympathizing with Karl Knutsson.

However, the Oxenstierna family lost power in Sweden in 1467 and in their stead Ivar Axelsson (Tott) was appointed as steward instead, by his brother, who was regent.

Ivar was married to Karl Knutsson's daughter Magdalena of Sweden, which meant a change of allegiance to the group that opposed the Union.

Ivar had his home in Visborg Castle on Gotland but nonetheless made major alterations to Stegeborg.

Ivar Axelsson owned seven vessels engaged in piracy on the Baltic Sea and this pastime led to an inevitable conflict with the Privy Council.

This eventually led to accusations from both bishop Brask and Nils Bosson (Grip) and he was one of those beheaded at Stockholm Bloodbath 1520th.

In 1622 the castle was conferred, along with some hundreds (härader) to Gustav II Adolf's brother-in-law John Casimir of Pfalz-Zweibrücken, who in 1651 was entitled Duke of Stegeborg.

After John Casimir's death in 1652 the ducal title passed on to the eldest living son, Karl Gustav.

On behalf of the Legal, Financial and Administrative Services Agency the castle was examined in 1728 but the governor's proposal was rejected.

In 1938 the Swedish National Heritage Board received a small sum to clear the location of trees and shrubs and the most severely damaged parts of the masonry were repaired.

During the years 1948–1955 the heritage board was assisted by national servicemen to excavate the ruins, repair the worst damage and raise protective roofing over certain parts.

The new main building (a private residence) is a palace, not a castle, and was completed in 1806, by the developer Werner Detloff von Schwerin.

[clarification needed] The palace got its present appearance, however, with the extensive renovations of 1915–1918, for Captain Hakon Wijk to designs by architect Isak Gustaf Clason.

A reflecting pool was built in front of the palace (today filled in) and a large orchard was designed by Danish landscape architect I. P. Andersen.

Stegeborg is owned today (2010) by the Danielsson family who operate a major tourist activity in the yard.