Horse Guards is a historic building in the City of Westminster, London, between Whitehall and Horse Guards Parade. It was built in the mid-18th century, replacing an earlier building, as a barracks and stables for the Household Cavalry. It was, between the early 18th century and 1858, the main military headquarters for the British Empire. Horse Guards originally formed the entrance to the Palace of Whitehall and later St James's Palace; for that reason it is still ceremonially defended by the King's Life Guard. Although still in military use, part of the building houses the Household Cavalry Museum which is open to the public. It also functions as a gateway between Whitehall and St James's Park. History The first Horse Guards building was commissioned by King Charles II in 1663, on the site of a cavalry stables which had been built on the tiltyard of the Palace of Whitehall during the Commonwealth. Built of red brick and costing some £4,000, it comprised a central range with a clock tower, under which an arch connected Whitehall with St James's Park; two wings enclosed a courtyard with two large sentry boxes for mounted troopers on the Whitehall side, facing the palace gate. Entry to the park, then an enclosed private garden, was controlled by special ivory passes issued to favoured courtiers, a tradition which continues to the present, although the modern passes are made of plastic; only the monarch has the right to drive through the arch without a pass. Initially, the building was intended only to accommodate the King's Guard and included stabling for more than a hundred cavalry horses on the ground floor, as well as separate barracks for the foot guards. Following a fire at Whitehall in 1698, the court transferred to St James's Palace, therefore the function of Horse Guards changed to controlling the ceremonial approach to St James's from Westminster. In the following decades, Horse Guards was increasingly used as administrative offices for the growing regular army and soon became overcrowded. The fabric of the building was also allowed to deteriorate; pieces of falling masonry were said to be a danger to the sentries. In 1745, King George II commissioned a new building in the fashionable Palladian style by the architect William Kent. Having to reuse the same plot of land, Kent managed to retain essentially the same plan as the original building while doubling the interior