Flat Holme Island forms part of the line of defence that lay across the Bristol Channel. Flat Holm was supported by Steep Holm Island and Lavernock Battery on the Welsh Mainland.
The four batteries on the Island were built between 1866 and 1871. A total of nine 7-inch RML guns were mounted in four batteries. All the guns were on Moncrieff Counterweight Pattern I Mounts, unlike Steep Holm where they were all on barbette mounts. The Batteries were:
Castle Rock Battery with three 7-inch RML guns in disappearing pits
Farm House Battery with two 7-inch RML guns in disappearing pits
Well Battery with one 7-inch RML gun in a disappearing pit
Lighthouse Battery with three 7-inch RML guns in disappearing pits.