Fort Bridgewood The site of Fort Bridgewoods is on the outskirts of Rochester , Medway in the United Kingdom , next to the Rochester - Maidstone road ( B2097 ) . The site was acquired by the War Office in about 1860 to form part of a ring of forts protecting the Royal Dockyard at Chatham . Building . It was envisaged that the line would stretch from the River Medway to the Thames but a shortage of money meant only five of the original large works plus two small experimental earthen redoubts were built . Work , using convict labour , started 30 years later in 1890 , but by that time the enemy it was supposed to repulse , France , was an ally , the new enemy was Imperial Germany . Because of budget restraints and changing fashions in fortifications , no fixed armament was mounted ; instead earthen ramps were built to enable field artillery to fire from the fort ’ s parapet . The fort was a radical departure from traditional design , of earth construction , with a deep dry moat designed to blend in with the line of the land . There were magazines and living quarters under the earthen walls . A new large prison was built on the hill above the nearby village of Borstal to house the workforce . It later became a prison for young offenders and gave its name to the Borstal Institution system of correction . Railway . Four of the forts ( Luton , Horsted , Bridgewoods and Borstal ) were linked by a railway , hauled by convicts , to move building materials between the sites . Building materials were brought by barge up the River Medway to a quay at Borstal ; they were landed , then hauled by a steam - powered ropeway up the steep scarp slope of the North Downs to Fort Borstal , where they were offloaded onto the railway . The prisoners were accompanied on their labours by armed warders on horseback . The railway remained in use until about 1905 . Some of the original railway lines can be seen in the cow - shed floor of the prison farm , which in a previous incarnation was the railway workshop . The line of a section of the track between Fort Bridgewood and Fort Horsted can be seen from the air ( for example on Google Earth ) , as it curves away in a south westerly direction from the junction of City Way and Marconi Way . World wars . After completion and until the outbreak of the First World War , Fort Bridgewoods was only sporadically garrisoned . In the early years of the 20th century the Royal Engineers deliberately mined and blew up one corner of the fort during a field exercise . With repairs undertaken the fort slipped into obscurity until trials with gun laying radar were undertaken at the Fort in the late 1930s . With radar installed , Bridgewoods served as headquarters for anti - aircraft guns in the Thames Estuary with the two flanking forts of Horsted and Borstal being among the most up - to - date anti - aircraft batteries in Britain . Nuclear age . With the development of the nuclear bomb , new protected headquarters were built at Bridgewoods in the early 1950s . Shortly after completion Anti Aircraft Command was disbanded and the site used for Civil Defence preparation and training . In 1960 the protected headquarters became the regional HQ for London ( south ) and remained in use until the early 1970s . In 1975 the site was sold to a property developer who quickly demolished the fort and bunker . The site remained empty for a number of years and is now a Parcelforce depot . Part of the fort is also under ' UK vending ' , whose address is Fort Bridgewood .