The Grand Shaft Barracks (Western Heights Barracks) were built during the Napoleonic Wars with a major staircase for troop movements between the barracks and defences in the town below. The stairs ruse 140 feet or 42 metres up to the Western Heights.
By 1896 the Barracks accommodated some 912 officers and men. Buildings included a gymnasium, stables and miniature rifle range built on the site of Drop Battery. See colour plan in the attached English Heritage Report.
This spiral staircase is unique in that it has three separate staircases, one to a well, another to a sally port and the third to Snargate Street. It is described as a triple helix staircase.
The Barracks have now all been lost, but the concrete platforms can still be seen.
The United Kingdom National Archives