Hidden Napoleonic Fortress.
Drop Redoubt Fort - Western Heights Dover Kent:
The name 'Drop' references the remnants of Dover's second roman lighthouse, or pharos, which is located on the redoubt. Locals know them as the 'Devil's Drop of Mortar' or Bredenstone.
The wars of 1779-83 saw the beginning of both fortifications on the two sites but what remains today dates from the early and middle 1800's. Construction began in 1804 in response to the threat posed to Britain by the Napoleonic Wars with France. The Drop Redoubt was built between 1804-8. A redoubt is a detached fort.
The Drop Redoubt is now owned by English Heritage and is sometimes open to the public.