Tower of London

Tower of London (処刑塔(ロンドンとう)) is a historic castle in central London, England. In the Toaru Majutsu no Index universe, it is still used to keep prisoners despite being a tourist attraction.

Principles
Historically, the facility was one known as a facility of blood, torture, and execution for prisoners. However, the Tower of London today is open to the public for just under 14 pounds. There, the facility displays historical execution devices as well treasures of the Royal Family.

Despite this, the facility is still being used for its original purpose due to the blind spots in the building. Prisoners were still kept there and they were tortured or executed without hesitation as needed. If one entered through the normal entrance, those shadowy portions were completely unreachable. If one entered through the secret entrance, one could not escape the shadowy portions. These blind spots, unlike the areas where the sightseeings were held, were cramped and dark.

Seven Tools of the Tower of London
Kanzaki Kaori mistakes the various tools of Misha Kreutzev as the Seven Tools of the Tower of London (処刑塔の七つ道具(ロンドンとうのななつどうぐ)) after looking at her. They are apparently famous interrogation tools used in the Tower of London. Since Kaori mistakes the tools on Misha's person as like that of the Tower of London, it can be assumed that the tools are pincers, hammers, an L-shaped crowbar, saw, and screw driver.

Academy City Invasion Arc
Lidvia Lorenzetti is mentioned being kept in the Tower of London.

Document of Constantine Arc
Stiyl Magnus along with Agnese Sanctis goes into the tower to interrogate Lidvia Lorenzetti and Biagio Busoni regarding God's Right Seat. While Biagio remains defiant Lidvia asks about what is happening outside. To which Stiyl says that she can make a guess on what was happening outside. She then shocks them with offering what she knows in exchange for have everyone that worked for her currently being held in the tower released.

Magic God Othinus Arc
Acqua of the Back comments about the second princess Carissa and the Knight Leader having both been held in the tower for their action involving the British Halloween following World War III and have been temporarily released for the emergency situation for a preemptive attack on GREMLIN.

Toaru Majutsu No Index SS: Norse Mythology
It was mentioned that if Kaori took out Leivinia Birdway and move her to the magical prison within the Tower of London, the world's safety will increase by a few percentage points.