Tre Kronor (Swedish pronunciation: [ˈtreː ˈkrûːnʊr]; "Three Crowns") was a castle located in Stockholm, Sweden, on the site where Stockholm Palace is today. It is believed to have been a citadel that Birger Jarl built into a royal castle in the middle of the 13th century. The name "Tre Kronor" is believed to have been given to the castle during the reign of King Magnus IV in the middle of the 14th century. Most of Sweden's national library and royal archives were destroyed when the castle burned down in 1697, making the country's early history unusually difficult to document.