Dover: Lock and Key of the Kingdom

The wars of 1779-83 saw the beginning of both of fortifications on  both of these sites but what remains today dates from the early and  mid 19th century. 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' refers to the remains of Dover's second Roman  lighthouse, referred to locally as the 'Devil's Drop of Mortar'). The  Citadel, a much larger fort, was still under construction in 1815 when  peace was declared and all work on the Western Heights ceased.  Deep defensive ditches lined with flint or brick surrounded both forts.  A renewed threat of war in Europe in the 1850s encouraged the  government to complete and modernise the fortifications at great  cost. What was described as 'a honeycomb in a hill' was built, capable of housing over 4,000 soldiers, whose role was to 'hide' in the forts  and attack any invader from the rear once they had passed Dover. Today the Drop Redoubt is owned by English Heritage and is  sometimes open to the public. The Citadel has most recently been  used as an Immigration Removal Centre which closed in 2015.