Vadstena Castle was originally built by King Gustav I in 1545 as a fortress to protect Stockholm from enemies approaching from the south.

The fortress consisted of three smaller stone buildings facing Lake Vättern, three 31-meter wide ramparts, a courtyard, a moat and four circular cannon turrets.

On August 22, 1552, King Gustav I married his third wife, Catherine Stenbock, in Vadstena.

The reconstruction from fortress into a habitable castle began in the 1550s, when Prince Magnus became duke of Östergötland.

Duke Magnus suffered from mental illness and was the only son of Gustav I who didn't eventually become king of Sweden.