North of Downing Street, on the site of the guard house of Whitehall Palace (which burned down in 1698) stands the 18th-century Horse Guards building, the headquarters of the Household Cavalry Mounted Regiment, a combination of the oldest and most senior regiments in the British Army -- the Life Guards and the Blues and Royals. Today these regiments have two principal duties: to protect the sovereign and to provide photo opportunities for tourists -- their dandy uniforms of red tunics and white plumed helmets for the Life Guards, blue tunics and red plumed helmets for the Blues and Royals, take a great shot.
The ceremony for changing the two mounted guards here is a good deal more accessible than the more famous one just down the road at Buckingham Palace. It takes place at 11am and 4pm from Monday to Saturday, and at 10am and 4pm on Sunday, and lasts around 30 minutes.
If you pass through the arch at Horse Guards, you'll find yourself at Horse Guards Parade, formerly the tiltyard (jousting area) of Whitehall Palace, which leads on to St. James's Park. It is here that the Household Cavalry help celebrate the Queen's birthday in June with a military pageant known as "Trooping the Colour".
Most of the building is usually closed to visitors, although a small section has been turned into the Household Cavalry Museum .