The Black Tower situated at the very east part of the Prague Castle represents the remains of the Romanesque rampart from the first half of the 12th century, the period of prince Soběslav I. The tower was built on the previous rampart grounds from the first half of the 11th century (the time period of Břetislav I). The tower also used to be called “Golden” at the times of the emperor Charles IV. This name was educed from the fact that the tower roof was covered by gold plate so it could give a signal from afar to those arriving that the estate of the powerful emperor is near.
From many view points the Black Tower seems to be a dominating feature. It has a square ground plan of almost 10m in length and its height reaches 40m. The width of the tower wall is admirable. The ground floor walls are 2.5m in width and as much as 3m on the north side. Initially the tower was primarily for defence. The top floor of the tower served as accommodation for the keeper, who monitored the surrounding area looking for possible fires or an enemy although it is not known whether any battle actually took place in front of the tower. The landscape simply did not allow that. The slope of Opyše and also the north and the south hillside were far too steep and the medieval ramparts were supported by moats.
In 1538 the tower was struck by lightening and burned down. At that time the tower roof was covered with burnt roofing. The extensive fires in Lesser Town, Prague Castle and Hradčany did not hit the tower directly but the tower walls were very sooty. And that is why the tower began being referred to as “Black”.
The tower used to serve as a prison. The roughest part of the prison, reserved for the real hard cases, was in the lowest part of the tower, a space created by enclosing the tower gate space. The Black Tower is not open to the public. Currently it houses a repository of archaeological treasures from Prague Castle.