"The complex at Beacon Hill could well be one of England's last but most significant unexplored historical landmarks"
Originally built in the 19th century, it saw use in World War One, and was re-adapted in World War II, retaining the same basic plan.
A new emplacement was built to the south and fitted with twin 6 pounder (2.7 kg) guns, used for fast moving targets such as torpedo boats.
Search lights were added to the 1860 battery on the front, and a pill box built on the extreme south west corner of the fort on Dovercourt Bay.
The fort contains six main gun emplacements, battery observation post, two pill boxes, shelters, accommodation for officers, guard hut, store, workshops, fire engine shed, magazines, electricians store and shop, engine room and electric light platforms.
The works continue along the coast northwards, comprising a massive bank into which pill boxes and a two storey hexagonal radar tower have been constructed, all with commanding views of the Orwell estuary opposite Landguard fort.
Many of the 20th century buildings survive and retain important military information on the architecture of gun installations, ammunition storage, communications, observation and radar posts.