The Bristol Channel Defences were often dealt with as one defence locality. This was particularly so when a defence line was built from Lavernock Battery in Wales, across the Bristol Channel to Breandown Battery, with the Islands of Flat Holm and Steep Home Islands covering the intervals. The Ports of Cardiff, Bristol and Swansea were all identified as having strategic importance.
The Home Ports Defence Committee, later the Home Defence Committee classed Cardiff and Swansea as Class C harbours with the following fixed defences:
- Swansea Port:
- Mumbles Battery with two 4.7-inch guns mounted in 1899 to 1901. This battery was built on the old tower that had been built between 1859 and 1861 with two casemated guns.
- Penarth Head Battery with two 6-inch Mark VII guns.
- Cardiff had two batteries:
- Nells Point Battery built between 1899 and 1900 with two 6-inch Mark VII guns
- Lavernock Battery was completed in 1870 and mounted four 7-inch RML guns. By 1903 the Fort had been rearmed with two 6-inch BL guns on HP mounts.
- Barry Port with Nell’s Point Battery.
- Flat Holm Island with four batteries mounting a total of nine 7-inch RML guns on disappearing mounts.
- Brean Down Fort with seven 7-inch RML guns.
- Steep Holm Island with ten 7-inch RML guns in six batteries.