The Grand Shaft is a unique triple staircase built in 1806-1809 to provide a short cut for troops from the Western Heights to the town. The shaft is 26 feet (8 metres) in diameter and 140 feet (42 metres) in height. It has three staircases of Purbeck limestone, which wind clockwise, one above the other, down a central brick light and ventilation shaft lit by an occasional window. At the bottom the three staircases meet in the sloping corridor which leads to Snargate Street. There are 200 steps in each staircase separated by several landings.Later, after the fear of invasion from France had subsided, the three staircases, which had initially been designed to allow the maximum number of troops to descend or ascend as quickly as possible, became segregated. Notices, which changed slightly over the years, were erected at the top of each staircase stating who was entitled to use which set of stairs. One of the most famous was:1. Officers and their ladies2. Sergeants and their wives 3. Soldiers and their women The shaft was restored in the 1980s.