Horse Guards Parade is a former tiltyard (enclosed courtyard for jousting) of Whitehall Palace. It lines three sides in Palladian style, was designed by William Kent and erected in 1750-60. 10 Downing Street backs onto the parade ground and the London Eye peeps over the top.
The entrance from Whitehall is guarded by two mounted cavalrymen of the Household division.
Every year on the Monarchs birthday the parade square is used to stage the trooping of the colour. Over 1,400 infantrymen and 200 mounted horsemen participate in a ceremony that dates back to a time when medieval commanders would parade their colours before battle as a rallying point for their soldiers. It's now a rallying point for tourists.
This building is part of the Ministry of Defence and the type of Government building James Bond stands on when contemplating the perils of previous missions.
The back yard of 10 Downing Street.