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, later becoming an important military headquarters. Horse Guards functions as a gatehouse giving access between Whitehall and St James's Park via gates on the ground floor. It 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 Queen's Life Guard. Although still in military use, part of the building houses the Household Cavalry Museum which is open to the public.