Fort Amherst Fort Amherst , in Medway , South East England , was constructed in 1756 at the southern end of the Brompton lines of defence to protect the southeastern approaches to Chatham Dockyard and the River Medway against a French invasion . Fort Amherst is now open as a visitor attraction throughout the year , running tours through the tunnel complex . The primary purpose of all the Medway fortifications was the defence of the Naval Dockyard . Land was acquired by two Acts of Parliament in 1708 and 1709 . The land was surveyed in 1715 by the Duke of Marlborough . The first plan of defences was an enceinte , from Gun Wharf , Chatham , to north of the village of Brompton . It was designed by Capt . John Desmoretze in 1755 and consisted of a 9m wide ditch and a 3m parapet . The strongpoint of the design was the Amherst Redoubt which became Fort Amherst . It was completed in 1758 and defended with 14 42 - pounders , 10 9 - pounders , 8 6 - pounders and 2 4 - pounder guns . These became known as the Cumberland lines , and were entered by four gateways with bridges . In 1770 Lt General Skinner ( who was a brother of Loyalist General Cortlandt Skinner , and both brothers were grandsons of Stephanus Van Cortlandt ) extended and strengthened the lines , the Amherst Redoubt was strengthened , further batteries added ( such as the Cornwallis Battery ) and the ditches revetted ( lined with brick ) . During the Napoleonic Wars the Chatham defences were enlarged and strengthened considerably . In 1802 - 11 prisoners were set to work on extending the tunnels and creating vast underground stores and shelters , new magazines , barracks , gun batteries and guardrooms . More than 50 smooth - bore cannon were also mounted . The last building works were about 1820 . A maze of tunnels dug into the chalk cliffs were used to move ammunition around the fort . A second gun battery , Townsend Redoubt , was built at the northeastern corner of the dockyard at the same time as Fort Amherst . Both forts were inside the 1756 brick - lined earthwork bastions known as the Cumberland Lines , which surrounded the whole east side of the dockyard down to St Mary ' s Island . These have now been built over . Fort Clarence in Rochester and Fort Pitt , on the Rochester - Chatham borders , were built in 1805 - 15 to protect the southern approaches . Although the Lines were never put to the test , its design would have made a formidable defence against any invasion force . In 1820 the defences were declared obsolete due to better artillery equipment with a greater firing range . The whole of the fortifications were used as a training ground during the Victorian period , the practice sieges were so popular that thousands of people came to Chatham to watch them . VIPs were given seating areas upon the Casemated Barracks that once stood in the Lower Lines and also upon Prince William ’ s Barracks within Fort Amherst . One of these sieges is described by Charles Dickens in his book ‘ Pickwick Papers ’ . Fort Amherst has been described by English Heritage as the most complete Napoleonic fortification in Britain and as such has great national historical significance . In 1959 , the site was scheduled as an ancient monument . The fort was still in use during the Second World War and restoration to make more areas open to the public was in progress . Current Usage . In the late 1970s a group of enthusiasts were given permission by the Ministry of Defence to start tidying up the site , with the intention of restoring Fort Amherst . In 1980 , the Fort was purchased from the MoD by the ' Fort Amherst and Lines Trust ' ( the original name of the trust ) and public open days began . Fort Amherst is managed and operated by the Fort Amherst Heritage Trust ( a charity ) , who open the fort to visitors on every day throughout the year , with tours running at 11am and 12pm every day . They also run a program of regular events including ghost tours and over night paranormal investigations through the tunnels . Halloween tours also take place , where guests make their way through the dark rooms and passages . Some of the cannons can be heard on Sundays throughout the year and periodically during school holidays . Ghost tours traditionally take place on the first Friday evening of each month . It was one of the filming locations for the 1986 Warner Bros . film " The Mission " . On Sundays , during the summer 2008 season , the Fort was open to the public , without charge , to promote the future Great Lines Heritage Park and the proposed World Heritage Site application for Chatham Dockyard and its Defences . 2012 marks the 200th anniversary of the establishment under Royal Warrant of the Royal Engineers at Brompton Barracks ( the Royal School of Military Engineering ) . Therefore a team of Royal Engineers designed , built and assembled the Bicentenary Bridge to mark this special anniversary . The bridge leads to a part of Fort Amhurst also known as Spur Battery and to the Inner Lines ( the Naploean defensive ditches ) . It is only accessible at special times and during guided visits . < br >