St James park & Horse guards Parade
A Royal Park, it is at the southernmost end of the St James's area, which was named after a once isolated medieval hospital dedicated to St James the Less, now the site of St James's Palace. The area was initially enclosed for a deer park near the Palace of Whitehall for King Henry VIII in the 1530's.
Horse Guards Parade was formerly the site of the Palace of Whitehall's tiltyard, where tournaments (including jousting) were held in the time of Henry VIII.
It was built in the mid-18th century, replacing an earlier building, as a barracks and stables for the Household Cavalry.