Tower of London
The oldest part of the Tower is the White Tower which was erected by, for, and at the command of, William I (otherwise William the Bastard, WIlliam of Normandy, or WIlliam the Conqueror) in the years after his conquest of 1066. Evidnece of the original fortification is now long-lost, but on the basis of other Norman conquest castles, it is likely to have been a wooden tower surrounded by a wooden palisade. Later, between 1080 and 1100, a stone tower, the White Tower, was built, using stone imported from Normandy, of unprecedented size, over 100 feet on each side and up to 90 feet tall.
Richard I (the Lionheart) ordered the further expansion of the fortress, leaving the execution of the work to William Longchamp, Bishop of Ely, his Chancellor, whilst he (the King) went off on crusade to Jerusalem (which, of course, he was fated never to reach). In 1238, after "difficulties" with his barons, Henry III ordered the building of stone curtain walls to the north and west and the replacemnt of the Roman wall on the east, as well as expanding the accommodation available within the walls and beginning to turn the Tower from a fortress simpliciter to a fortified residence.
Between 1275 and 1285 Edward I transformed the Tower by causing great concentic walls (then the fashion in fortification) to be thrown up, as well as filling in the existing moat and creating a new one. Under this Edward and his son, the Tower came to be both a repository for Royal records, and also the site of the Royal Mint (used to season the Royal lamb -- not).