At the top of Box Hill, one of the highest points in Surrey, there is a Victorian fort, believed to be one of the earliest reinforced concrete constructs built. It was built in the 1890's to protect London from invasion from the continent.
One of 13 mobilisation centres forming the London Defense Positions, it was designed to store ammunition in tunnels and the doors and windows are cannon-proof for protection. It is surrounded by ramparts and trenches from which the infantry would fight.
None of the forts were ever used, and bats have now taken up resident in this one, which means it is not open to the public.
My entry to the last of this season's Derelict Thursday Challenge