Harwich Redoubt Harwich Redoubt is a circular fort built in 1808 to defend the port of Harwich , Essex from Napoleonic invasion . The Harwich Society opens it to the public . History . Construction . The Redoubt was built between 1808 and 1810 to protect the port of Harwich against the threat of Napoleonic invasion . It was part of the scheme that included the construction of 29 Martello Towers on the East Anglian coast . The Redoubt is of circular shape , approximately 200 ft in diameter , with a central parade ground of 85 ft diameter . Hoists were used to lift shells from the lower level to the gun emplacements . It is similar in design to earlier redoubts at Dymchurch and Eastbourne . Though difficult to imagine as it is now surrounded by houses , when the Redoubt was built it was on a hill top with free views in all directions . A house was demolished to make way for the Redoubt , and a large elm tree - used by ships as a navigational mark - was also removed . It is said that French prisoners of war were made to help in the construction . Military service . Originally armed with ten 24 - pounder cannon , the Redoubt was remodelled in order to accommodate increasingly heavy guns , as technology and the perceived threat changed . In 1861 - 2 , work was carried out to accommodate 68 - pounder cannon , and the emplacements were strengthened ( by adding granite facing ) to withstand improved enemy artillery . Only a decade later in 1872 , three of the emplacements were altered to take enormous 12 ton RML ( Rifled Muzzle Loading ) guns . In 1903 , three emplacements received 12 pounder QF ( quick firing ) guns . Despite this ongoing modernisation , the Redoubt never fired a shot in anger . It is also probable that its strategic importance declined towards the end of the 19th century with the construction of the more powerful Beacon Hill Battery just to the south . In the 1920s the area around the Redoubt - previously kept clear to provide fields of fire - was bought by the Town Council . This land is used for allotments . The Redoubt itself was allowed to fall into disrepair . The Redoubt was briefly taken back into military service during World War II , when it served as a detention centre for British troops awaiting trial . Examples of the graffiti left by the soldiers can still be seen in some of the rooms . Following World War II the Redoubt was used by the British Civil defence organisation who used it until they were disbanded . That was the end of the Redoubt ' s military service . Restoration . Following a listing as an Ancient Monument , the restoration of the Harwich Redoubt began in July 1969 and has continued until the present day . When restoration began , one of the 12 ton RML guns was discovered buried in the ( dry ) moat - it had been pushed in when it became obsolete , as they were too large and solidly constructed to be broken up for scrap . Currently open as a museum , the Redoubt is the largest ancient monument in the UK being restored by a voluntary group . In addition to some of the original guns that armed the Redoubt , including the aforementioned 12 ton RML , there are a number of other guns on display . Around the parade ground on the lower level , the various rooms are either fitted out in reconstruction of how they might have looked , or are used as display rooms for other exhibitions . Exhibitions include a large number of replica firearm s . Battle re - enactments and other events are held during the summer months .