Admiralty Citadel: What Is The Concrete Box On The Mall?

By M@ Last edited 13 months ago

Last Updated 19 May 2023

Admiralty Citadel: What Is The Concrete Box On The Mall?
The ivy-covered Admiralty citadel

The mysterious concrete box with a secret history.

You know The Mall. It's all posh, stuccoed buildings, park and palace. And then there's this thing:

The bare side of the admiralty citadel, on the mall

The box of concrete sticks out from the corner of Horse Guards like a sore thumb. Which is ironic, because it was once the hub of Britain's top secrets. This is the Admiralty Citadel, and it's gained the oxymoronic achievement of becoming a covert landmark.

Bomb-proof comms hub

Many Londoners will be familiar with the structure, thanks to its sheer oddness. But its age and purpose are often misrepresented. You'd be forgiven for thinking that the citadel was a Cold War bunker. Its thick walls, built from reinforced concrete with pebble and flint aggregate blocks, look like they were designed to stand up to mushroom clouds. But the structure predates the nuclear era.

The Admiralty Citadel was actually built at the very beginning of the second world war, to serve as a bomb-proof command centre for the Royal Navy. Military ciphers would come and go from here — the cutting edge of encryption and telecommunications within an essentially medieval-style fortification. It housed up to 1,000 staff, beavering away over teleprinters in 100 separate rooms.

The whole package was protected from bombing by those chunky walls and a six-metre-thick roof. It was also designed for siege, complete with gun emplacements and watch tower. This building would have been the military's last point of defence had a ground invasion occurred. The complex was hooked up to other bunkers and Whitehall buildings via a series of tunnels. This is proper 'secret London'.

"A vast monstrosity"

A side view of the Admiralty citadel

The building served its purpose well but, after the war, came in for regular criticism. Churchill famously described it as a "vast monstrosity" when set against the elegant buildings of the area. In 1955, the House of Commons debated what to do with the cubist fortress. Demolition was not an easy option. Pulverising the thick walls and deep foundations would have been a costly endeavour. Cladding the behemoth in Portland stone was also ruled out on the grounds of cost.

The only practical measure was to encourage the growth of creepers (a solution which one MP feared "would look a bit like an inverted crew-cut along the top of the building". Some foliage was already present, including a then-rare roof garden, first installed to disguise the facility from the air. By 1948, one might have seen "red and pink hollyhocks, gladioli and marigolds waving about the ramparts".

A wall of ivy on the admiralty citadel

Today, the fortress is almost entirely covered in ivy and other plants, whose appearance changes through the seasons. It would be easy enough to scale the walls, were it not for the bank of security cameras covering every inch.

The building has found low-key uses over the years, reportedly as a navy training centre, storage facility, and latterly as a data centre. It is still owned by the Ministry of Defence, and once again serves as a communications centre, officially known as MARCOMM COMCEN (St Vincent).

Occasionally, part of the roof is called into service as a seating area for important events. This includes the coronation procession of Charles III. Short of being in the Abbey itself, this would have to be one of the best seats from which to witness the event.

Balcony seats on the admiralty citadel

Despite its ugly beginnings, the Admiralty Citadel has matured into a not altogether unattractive bank of green walls (at least in summer). As early as 1987, the building was granted a Grade II-listing, on account of its historical importance. Perhaps one day, somebody will find an imaginative use for this unique structure, and its 100 rooms will spring to life once again.

All images by Matt Brown. (Unless the security services are asking, in which case someone else took them.)