The Admiralty Citadel, London's most visible military citadel, is located just behind the Admiralty building on Horse Guards Parade. It was constructed in 1940–1941 as a bomb-proof operations centre for the Admiralty, with foundations 30 feet (nine metres) deep and a concrete roof 20 feet (six metres) thick. It is also linked by tunnels to government buildings in Whitehall.[6]
Sir Winston Churchill described it in his memoirs as a "vast monstrosity which weighs upon the Horse Guards Parade" – and Russian vine[6] has been encouraged to cover it in an apparent attempt to soften its harsh appearance. Its brutal functionality speaks of a very practical purpose; in the event of a German invasion, it was intended that the building would become a fortress, with loopholed firing positions provided to fend off attackers.
In 1992 the Admiralty communications centre was established here as the stone frigate HMS St Vincent, which became MARCOMM COMCEN (St Vincent) in 1998. The Admiralty Citadel is still used today by the Ministry of Defence.
It's not as old as it looks.