Yesterday, I attended with some friends the Ceremony of the Keys at the Tower of London. It is the oldest ongoing ceremony in the world, carried out, without fail, for nearly 750 years.
The Ceremony lasts only for seven minutes, commencing at exactly 9.53 pm. Escorted by Foot Guards, the Chief Yeoman Warder (who must have served in the armed forces for more than 20 years) locks first the Middle Tower Gate (39) and then the Byward Tower Gate (38). Once locked, none of the gates can be opened from the outside, even if someone had acquired a set of the Keys.