The ruins lie beneath Christiansborg Palace in Copenhagen and show the remains of Absalon's Castle from the 12th century and Copenhagen Castle from 1369. The ring wall of Absalon's Castle, built of limestone shipped from the Cliffs of Stevns, enclosed an area about 53 metres square. Valdemar Atterdag built Copenhagen Castle on top of the remains of the destroyed fortress. The site also includes the Blue Tower (Blåtårn) prison, where Leonora Christina was held for 21 years, 9 months and 11 days, and the foundation stone of the present palace is visible deep underneath.
The museum is run by the Royal Danish Collection. The cellars are dark, and visitors get close to the ruins. There is a gift shop.
