Westminster’s grand Old Admiralty Building is the backdrop to many of the nation’s most important ceremonial events. Its turrets and handsome red brick and Portland stone facades face both The Mall and Horse Guards Parade, scene of Trooping the Colour and the Olympic beach volleyball in 2012. This summer, on the 75th anniversary of VJ Day, the actor Hugh Bonneville clambered through a window while vintage newsreel was beamed across the facade.

These 130-year-old walls, enclosing almost a hectare of prime land in a highly secure area of Whitehall, are all that most people will ever see. But it is within them that some of the most significant – and classified – events in the 20th-century history of the UK have taken place.

It was from here that Winston Churchill, as First Lord of the Admiralty, waged a naval arms race against the German empire in the run-up to the First World War. It was also here, in 1917, that cryptographers cracked the Zimmermann Telegram, changing the course of that war.

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