In return, they had a room in the tower, and they received from the King the authorization to make three wide gothic openings in the wall. But they had to be blocked up in case of war.
The Bishop's Tower is not cylindrical but square-shaped and blocks the way between the Higher and the Lower Lists. Corner turrets and a watch path complete the defence system. Going from one side of the fortifications to the other, there is no other possibility than passing under its vaults.
This tower is stepped over the bailey, thus creating a very narrow passage that could easily be controlled.
Viollet le Duc, the famous architect who saved the Cité from total destruction in the 19th century, installed his office in this tower during the restoration works.