Restored by the Landmark Trust, it is now home to several small businesses, museums, exhibitions and a holiday apartment sleeping up to eight people.
It was built 400 metres in front of the defensive line, now incorporated into the east-west Crownhill Road, in an exposed position on a natural outcrop.
Crownhill Fort is designed for all round defence, with each of its seven sides having massive ramparts and being surrounded by a deep ditch.
In 1866, after a strike, George Baker, the building contractor, went bankrupt and so the work was finished by the Royal Engineers who would become Crownhill Fort's first occupants.
None of the original armament survives but The Landmark Trust has reinstated several artillery pieces similar to those that would have occupied Forts of this kind.
In the First World War, Crownhill Fort was used as a recruitment and transport centre for troops being sent to the fronts in Turkey and Africa.
Visitors can see museum displays, watch demonstrations of Victorian firepower and take guided tours in the labyrinth of underground rooms and passages.