In 1865 The War Office purchased ten acres of land on Flat Holm; four batteries were built during the military occupation of 1866 to 1903:
1. Well Battery: had one open-backed Moncrieff gun-pit.
2. Farmhouse Battery: retains two Moncrieff gun-pits set under a common mound with a magazine between them.
3. Castle Rock Battery: Three Moncrieff gun-pits and magazine with integrated earthen banks running back behind it to protect the landward approaches.
4. Lighthouse Battery and Fort: Three Moncrieff gun-pits integrated with the defences enclosing the barracks, administrative buildings, and pre-existing lighthouse.
Flat Holm was re-occupied by the military during the Second World War and developed as an important and active anti-aircraft installation (nprn 407348).
RCAHMW, 20 July 2010.