
Flat Holm Island (Flat Holme) was fortified in the early 1860s as part of the defence line that stretched across the Bristol Channel between Brean Down Fort and Lavernock Battery.
A barracks was built on the the southern end of the island, adjacent to the lighthouse. A dry moat was built around this area to make it a position of last defence in the event of an enemy landing.
Lighthouse Battery was built immediately adjacent to the Barracks.