A number of military citadels are known to have been constructed underground in central London, dating mostly from the Second World War and the Cold War. Unlike traditional above-ground citadels, these sites are primarily secure centres for defence coordination.

A large network of tunnels exists below London for a variety of communications, civil defence and military purposes; however, it is unclear how these tunnels, and the various facilities linked to them, fit together, if at all. Even the number and nature of these facilities is unclear; only a few have been officially admitted to.

Ministry of Defence citadels

The Ministry of Defence (MOD) Main Building in Whitehall was outfitted with two bunkers, known as the North and South Citadels, when first built. The site of the South Citadel was later used for an improved "Defence Crisis Management Centre" bunker. The bunker is named after the ancient Greek poet Pindar whose house was the only one left standing in Thebes following the city's destruction in 335 BC.

Planning for the Pindar bunker began in 1979, with ministerial approval being granted in November 1982 The bunker cost £126.3 million, £66.3 million of which was spent on the civil engineering element;

Pindar has two floors; the lower floor contains the Ministry of Defence's Joint Operations Centre (previously situated on the fifth floor of the MOD Main Building), and the upper floor consists of Government Emergency Rooms (comprising the Prime Minister, Secretaries of State, the Cabinet Secretary, and some Permanent Secretaries), an element of the Joint Intelligence Organisation, and a telecommunications secretariat and a Cabinet Office Communications Centre (COMCEN) element. the tunnel predated the bunker and was already used as a conduit between the Cabinet Office and the MOD Main Building, with Downing Street access being added during Pindar's construction. When answering written questions about Pindar, which included a question on the extent of lift and staircase access to the bunker and on whether there was any connection to transport systems, then-Armed Forces Minister Jeremy Hanley would say only that there were "sufficient means of access and egress" and denied that the bunker was connected to any transport system; he also said that there were means of leaving Pindar should the MOD Main Building collapse on top of it, but did not state the details of these. the British photographer David Moore carried out an extensive photographic survey of an underground facility that was widely believed (and strongly hinted) to be Pindar, and the Ministry of Defence stating in later years that Pindar was indeed the facility depicted in the photographs. The photographs were published as The Last Things in 2008 and 2009.

In addition to the bunker under the Main Building, it was intended that a reserve location be established at the site of the Kingsway telephone exchange in High Holborn; A further change in intentions relates to the "COBR" Cabinet Office Briefing Rooms; while it was intended that Pindar would replace these,

Admiralty Citadel

thumb|The Admiralty Citadel in 2008

The Admiralty Citadel, London's most visible military citadel, is located just behind the Admiralty building on Horse Guards Parade. It was constructed in 1939 by the Ministry of Works with the architect W. A. Forsyth as a consultant. It was designed as a bomb-proof operations centre for the Admiralty, with foundations deep and a thick concrete roof. It is also linked by tunnels to government buildings in Whitehall.

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. Sir Winston Churchill described it in his memoirs as a "vast monstrosity which weighs upon the Horse Guards Parade". It became a Grade II listed building in December 1987.

Q-Whitehall and tunnel network

Q-Whitehall is the name given to a communications facility under Whitehall.

The facility was built in a 12 ft (3.7 m) diameter tunnel during World War II, and extends under Whitehall. The project was known as 'Post Office scheme 2845'. A detailed description, with photographs, was published just after the war in the January 1946 edition of The Post Office Electrical Engineers' Journal.

Sites equipped with unusual amounts of GPO/BT telecommunications plant are given a BT site engineering code. This site's code was L/QWHI.

The site provided protected accommodation for the lines and terminal equipment serving the most important government departments, civil and military, to ensure the command and control of the war could continue despite heavy bombing of London.

At the northern end, a tunnel connects to a shaft up to the former Trafalgar Square tube station (now merged with Charing Cross station), and to the BT deep level cable tunnels which were built under much of London during the Cold War. At the southern end, an 8 ft (2.4 m) diameter extension (Scheme 2845A) connects to a shaft under Court 6 of the Treasury Building: this provided the protected route from the Cabinet War Room. This was known as Y-Whitehall. The tunnel was further extended (Scheme 2845B) to the Marsham Street Rotundas. This extension housed the 'Federal' telephone exchange which had a dialling code of 333 from the public network. In the 1980s it housed Horseferry Tandem which provided a unified communications system for all government departments as well as the Palace of Westminster.

Access to the tunnel is gained via an 8 ft (2.4 m) lateral tunnel and a lift shaft in the nearby Whitehall telephone exchange in Craig's Court. A further entrance is via the deep level portion of the Admiralty.

Spur tunnels, 5 ft (1.5 m) in diameter, were built to provide protected cable routes to the major service buildings either side of Whitehall.

The Whitehall tunnels appear to have been extended in the early 1950s. Some official documents refer to a Scheme 3245: and created a map of both this network and the deep level cable network based on his investigation. Campbell did not find the network to be particularly secure; many of the access shafts could be easily found and entered by unauthorised individuals such as himself, and while patrolmen were present throughout the tunnels they were more concerned with checking structural integrity than stopping intruders.