Drop Redoubt Fort, Western Heights, from the ditch
Drop Redoubt is a fort on the Western Heights, Dover. The construction of the Redoubt was in two periods, the first being from 1804-1808 during the Napoleonic Wars, the second from 1859-1864 following the recommendations of the 1859 Royal Commission.
These images are from the ditch point of view. Apparently forts have ditches, castles have moats mostly filled with water