Portland's Breakwater, or 'Chequered' fort dates from c1860 and became locally known as "Palmerston's Folly".
The fort was intended as a casemented granite construction, following the 1859 Commission Report on the Defence of England, however due to subsidence, it was eventually constructed in Portland stone, concrete and armour-plated steel, astride a granite base. This was constructed on a 200-foot (61 m) diameter ring of stone laid down on the seabed.
The Breakwater Fort was built between 1868 and 1879 but considerably modified and extended in 1884, 1900, 1912 and later. Additional armaments were added in both World Wars.
Information by kind permission of Portland History © Ashley Smith