Hidden inside the south‑west tower of St Paul’s Cathedral lies Sir Christopher Wren’s Geometric (Dean’s) Staircase. This self‑supporting spiral of 88 stone steps rises without a central column and forms a perfect helix of white Portland stone. Built in the early 1700s, it is considered one of Wren’s finest engineering feats and was once used by cathedral officials to reach the triforium. The staircase is rarely open to casual visitors; it can be seen only on special guided tours, adding to its mystique.