Blåtårn, which could be translated as "Blue Tower", was the greatest tower of Copenhagen Castle, and its most fearsome prison. The tower was build sometime in the 15th century and eventually demolished in 1731.

During its three century long existence, Blåtårn housed many political prisoners. Amongst these were female members of the Swedish House of Vasa, imprisoned shortly after the Stockholm Bloodbath instigated by King Christian II. Following the deposition of Christian II in 1523, Denmark and Sweden made peace and the prisoners were released. 

The most famous prisoner was the noblewoman Leonara Christina Ulfeldt. Her husband Count Corfitz Ulfeldt was the most notorious traitor in Danish history.

Copenhagen Castle was teared down in 1731 along with Blåtårn, and Christiansborg Palace was built where Copenhagen Castle and Blåtårn once stood. The 106 metre high tower of the current third Christiansborg Palace, completed in 1928, has a strong resemblance to Blåtårn.