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 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.