This configuration of spiral staircase is a fairly common sight in American starforts (Fort Massachusetts and Fort Popham spring to mind). A more conventional staircase seems as though it would be a simpler way to ascend to a fort's terreplein, but space in a starfort was always at a premium, and the spiral staircase occurred in nature more frequently in the 19th century than it does today. Fort Knox boasts two spiral staircases!