Fort Cumberland ( England ) Fort Cumberland is a pentagon al artillery fortification erected to guard the entrance to Langstone Harbour , east of the naval port of Portsmouth on the south coast of England . It was sited to protect the Royal Navy Dockyard , by preventing enemy forces from landing in Langstone Harbour and attacking from the landward side . Fort Cumberland is widely recognised as the finest example of a bastion trace fort in England . The first fort ( 1747 - 1785 ) . The first fort on the site was built by the Duke of Cumberland . Although there did previously exist an earthwork battery on the site , built in 1714 . Work on Cumberland ' s fort commenced on January 1 , 1747 , and was substantially complete by the end of 1748 . The fort had an irregular star shaped form , and was of earthwork construction . In section , the defences comprised a glacis , banquette and covered way , a dry ditch , berm , rampart , parapet and terre - plein . Within the body of the fort , a number of brick buildings were constructed , comprising a guardhouse , storeroom and powder magazine ; of the two proposed barrack blocks , only one is believed to have been completed . The second fort ( 1785 - 1812 ) . Fort Cumberland was completely rebuilt in masonry , and on a considerably larger scale , as part of a programme to improve the fortifications of Portsmouth . Major building work started in 1785 , although preliminary stockpiling of materials began in 1782 , and was essentially completed by 1812 . In the course of construction all traces of the original fort were destroyed , with the exception of the guard house and store room , both of which were incorporated into the new design . The second fort occupies a similar position to the first , but is substantially larger in scale , enclosing an area of , and on a different alignment . It is a polygonal fort , with five bastion s and a ravelin on the western side . It was the last fort with angle bastions to be built in the United Kingdom , and the first to provide casemate d ordnance . Re - armament : the 1850s - 1860s . By the late 1850s , the development of rifled cannon had rendered the fort ' s smooth bored muzzle loading ordnance obsolete . In late 1858 the fort was transferred from the War Department to the Board of Admiralty and , in 1859 it became the headquarters of the Royal Marine Artillery . Between 1860 and 1861 , the defences were improved to accommodate new muzzle loaded Armstrong guns . Re - armament : the 1880s - 1890s . Between 1886 and 1892 , major modifications were made to three of the bastions to accommodate new armaments , 6inch B . L . MkIV guns on Mark IV Hydropneumatic Disappearing mountings . These modifications included cutting down the bastions and infilling them internally , and the creation of new magazines underneath . By 1892 , the fort was equipped with three BLs , two RMLs and 1 64 pounder . The fort in the 20th century . Fort Cumberland remained in military ownership for most of the twentieth century , serving as a base for the Royal Marine Artillery howitzer and anti aircraft brigade , and later for the Royal Marine Mobile Naval Base Defence Organisation , as an experimental and training centre . Beginning in 1938 , Fort Cumberland also provided space for the Inter - Service Training and Development Centre . The fort saw brief action during the Second World War when , on August 26 , 1940 , it was hit by a German air raid in which eight Royal Marines were killed . In 1964 the fort was scheduled as an ancient monument , and subsequently taken into the guardianship of English Heritage in 1975 . Since that time , the fort has served as a base for English Heritage ' s archaeological team , the Central Archaeology Service ( formerly the Central Excavation Unit ) . In 1998 , the Ancient Monuments Laboratory were relocated to the fort , leading to the creation of a new Centre for Archaeology . Currently access is limited to pre - booked guided tours .