Blåtårn (in English: Blue Tower) was a tower in Copenhagen Castle, the Danish royal family's palace in Copenhagen, Denmark. The tower was used as a dungeon and has been known as such in history. It existed from at least the late 15th century until 1731.
It is not known when the tower was built, but it is known to have existed during the reign of King John I of Denmark (reign 1481-1513).
Many famous people were imprisoned in Blåtårn during its history. In the 1520s, a large number of Swedish prisoners were taken there after the Swedish rebellion against Denmark and the subsequent declaration of Swedish independence, notably the renowned heroine Christina Gyllenstierna and female members of the House of Vasa, Sweden's ruling dynasty. Perhaps its best known prisoner was Leonora Christina Ulfeldt (a daughter of Christian IV of Denmark and wife of the Danish statesman-cum-traitor, Corfitz Ulfeldt), who was imprisoned here at the behest of the Queen-Mother, Sophia of Brunswick, between 1663 and 1685, during which she wrote several works, including her famous autobiography Jammers Minde.
The tower was destroyed in 1731-32. After this, the same name was also used for a gaol in Copenhagen near Frederiksholm channel; that building was torn down in 1848. A tower of Sønderborg Castle, where the deposed king Christian II of Denmark was imprisoned, has also been known by this name, but was demolished in 1755.
Famous prisoners of Blåtårn
- Margareta Eriksdotter (Vasa) sister of king Gustav I of Sweden.
- Cecilia Månsdotter mother of king Gustav I of Sweden.
- Sigrid Eskilsdotter (Banér) grandmother of king Gustav I of Sweden.
- Christina Gyllenstierna, the defender of Stockholm
- Märta Eriksdotter (Vasa) sister of king Gustav I of Sweden.
- Emerentia Eriksdotter (Vasa), sister of king Gustav I of Sweden.
- Torben Oxe,
- Knud Pedersen Gyldenstjerne, councillor of state
- Joachim Rønnov, bishop
- Leonora Christina Ulfeldt, daughter of king Christian IV of Denmark