St Catherine's Island, off Tenby

You get to Island at low tide across the sand and up a steep path to the Victorian fort at the top. In 2018 it was open to visitors, although the future is uncertain.
At the top of the steps is an iIrregular plan concrete gun floor with centrally set steel holdfast for 4-inch Mk IX QF gun, enclosed by a low concrete wall. Magazine located in pre-existing, rectangular plan, limestone rubble, secondary flat reinforced concrete roofed structure.

Google view of the island, showing the location of the gun platform