Victorious at the Battle of Hastings on 14 October 1066, the invading Duke of Normandy, William the Conqueror, spent the rest of the year securing his holdings, by fortifying key positions. He founded several castles along the way, but took a circuitous route toward London; only when he reached Canterbury did he turn towards England's largest city. As the fortified bridge into London was held by Saxon troops, he decided instead to ravage Southwark before continuing his journey around southern England. A series of Norman victories along the route cut the city's supply lines and in December 1066, isolated and intimidated, its leaders yielded London without a fight. Between 1066 and 1087, William established 36 castles, although references in the Domesday Book indicate that many more were founded by his subordinates. The new ruling elite undertook what has been described as "the most extensive and concentrated programme of castle-building in the whole history of feudal Europe". They were multi-purpose buildings, serving as fortifications (used as a base of operations in enemy territory), centres of administration, and residences.

William sent an advance party to prepare the city for his entrance, to celebrate his victory and found a castle; in the words of William's biographer, William of Poitiers, "certain fortifications were completed in the city against the restlessness of the huge and brutal populace. For he [William] realised that it was of the first importance to overawe the Londoners".At the time, London was the largest town in England; the foundation of Westminster Abbey and the old Palace of Westminster under Edward the Confessor had marked it as a centre of governance, and with a prosperous port it was important for the Normans to establish control over the settlement. The other two castles in London – Baynard's Castle and Montfichet's Castle – were established at the same time. The fortification that would later become known as the Tower of London was built onto the south-east corner of the Roman town walls, using them as prefabricated defences, with the River Thames providing additional protection from the south. This earliest phase of the castle would have been enclosed by a ditch and defended by a timber palisade, and probably had accommodation suitable for William.

The Tower of London, officially Her Majesty's Royal Palace and Fortress of the Tower of London, is a historic castle located on the north bank of the River Thames in central London. It lies within the London Borough of Tower Hamlets, separated from the eastern edge of the square mile of the City of London by the open space known as Tower Hill. It was founded towards the end of 1066 as part of the Norman Conquest of England. The White Tower, which gives the entire castle its name, was built by William the Conqueror in 1078, and was a resented symbol of oppression, inflicted upon London by the new ruling elite. The castle was used as a prison from 1100 (Ranulf Flambard) until 1952 (Kray twins), although that was not its primary purpose. A grand palace early in its history, it served as a royal residence. As a whole, the Tower is a complex of several buildings set within two concentric rings of defensive walls and a moat. There were several phases of expansion, mainly under Kings Richard the Lionheart, Henry 3, and Edward 1 in the 12th and 13th centuries. The general layout established by the late 13th century remains despite later activity on the site.