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. The current and previous buildings were, 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.
Wikipedia https://en.wikipedia.org/wiki/Horse_Guards_(building)
Coordinates 51°30'17.113" N -0°7'37.409" E