The Constable of the Tower is the most senior appointment at the Tower of London. In the Middle Ages a constable was the person in charge of a castle when the owner - the king or a nobleman - was not in residence. The Constable of the Tower had a unique importance as the person in charge of the principal fortress defending the capital city of England.
Today the role of Constable is a ceremonial one and mainly involves taking part in traditional ceremonies within the Tower as well as being part of the community that lives within its parameters. The office is currently held by General The Lord Dannatt GCB CBE MC, who assumed the post on 1 August 2009. On 20 July 2012, the Constable welcomed the Olympic Torch to London at the Tower one week in advance of the London 2012 Summer Olympic Games, as part of the Olympic torch relay.
He is the 159th Constable; although the precise number of Constables is uncertain, but 159 is the number arrived at after an estimate during the last century.
Under the Queen's Regulations for the Army, the office of Constable is conferred upon a Field Marshal or a retired General officer for a five-year term.
At the conclusion of the Constable's Installation ceremony, the Lord Chamberlain symbolically hands over the Queen’s House to the Constable. He in turn entrusts it to the Resident Governor, who is responsible for the day-to-day running of Her Majesty’s Palace and Fortress, the Tower of London.
More information on the Wikipedia page