St. Catherine's Island, Tenby, Pembrokeshire.
St Catherine's Island, is a small tidal island linked to Tenby in Pembrokeshire, Wales, by a beach at low tide. The island is home to a fort, constructed to protect Pembroke Dock and completed in 1870. In 1907, the island was sold privately for 500 pounds. St. Catherine's Fort has four main bedrooms, 16 turret rooms and an old banquet hall with a life size statue of Queen Victoria and an old, half-broken suit of armour. In the basement there is an old armoury which used to hold 444 barrels of gunpowder. At one time the fort was used as a zoo.
Info text from coletracey
More info and photos of the inside of this amazing place here