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The Tower of London, officially His Majesty's Royal Palace and Fortress of the Tower of London, is a historic citadel and castle on the north bank of the River Thames in London, England. It lies within the London Borough of Tower Hamlets, which is 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 toward the end of 1066 as part of the Norman Conquest. The White Tower, which gives the castle its name, was built by William the Conqueror in 1078 and was initially a resented symbol of oppression. A grand palace early in its history, it served as a royal residence. 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 I, Henry III, and Edward I in the 12th and 13th centuries.

The Tower of London was besieged several times, and controlling it has been important to controlling the country. The Tower has served as an armoury, a treasury, a menagerie, the home of the Royal Mint, a public record office, and the home of the Crown Jewels of England. From the early 14th century until the reign of Charles II in the 17th century, the monarch would traditionally prepare for several nights at the Tower, and lead a procession from there to Westminster Abbey for their coronation. In the late 15th century, the Princes in the Tower were housed at the castle when they mysteriously disappeared, presumed murdered. Under the Tudors, the Tower was used less as a royal residence, and despite attempts to refortify and repair the castle, its defences lagged behind developments to deal with artillery.

The zenith of the castle's use as a prison was the 16th and 17th centuries, when many figures who had fallen into disgrace, such as Elizabeth I before she became queen, Sir Walter Raleigh, and Elizabeth Throckmorton, were held within its walls. This use has led to the phrase "sent to the Tower". Despite its enduring reputation as a place of torture and death, popularised by 16th-century religious propagandists and 19th-century writers, only seven people were executed within the Tower before the world wars of the 20th century. Executions were more commonly held on Tower Hill to the north of the castle, with 112 occurring there over a 400-year period. In the latter half of the 19th century, institutions such as the Royal Mint moved out of the castle to other locations, leaving many buildings empty. Anthony Salvin and John Taylor took the opportunity to restore the Tower to what was felt to be its medieval appearance.

In the First and Second World Wars, the Tower was again used as a prison and witnessed the executions of 12 men for espionage. After the Second World War, damage caused during the Blitz was repaired, and the castle reopened to the public. Today, the Tower of London is the country's ninth most popular tourist attraction, with 2,817,852 visitors in 2025. Under the ceremonial charge of the Constable of the Tower, operated by the Resident Governor of the Tower of London and Keeper of the Jewel House, and guarded by the Yeomen Warders, the property is cared for by the charity Historic Royal Palaces and is protected as a World Heritage Site.