The Cabinet War Rooms were not obvious, to prevent civilians and spies from camping out in front. They were built into government buildings in central London. The street was closed off during the war, so unless you knew they were there, it would be hard to realize something important was there. Right-click and choose "view image" to enlarge.
Welcome to the Imperial War Museum The Imperial War Museum, the multi-branch national museum of war and wartime life from the First World War, through the Second World War, to the present day.