Which historic castle in London, a World Heritage Site, was a royal prison?

The historic castle in London that was a royal prison and is a World Heritage Site is the Tower of London.

Founded after the Norman Conquest, the Tower's central White Tower was begun by William the Conqueror around 1078. It served as a fortress, royal residence, treasury, armoury and menagerie, but its role as a prison made it especially famous. Elizabeth I, Sir Walter Raleigh and Guy Fawkes were among those confined there.

The Tower was not a prison for ordinary criminals, and many prisoners were held there briefly because of their political importance. Its reputation for torture and constant executions was amplified by later propaganda and popular literature. Most executions associated with the Tower actually took place on nearby Tower Hill.

UNESCO added the Tower to the World Heritage List in 1988. Today it is also home to the Crown Jewels, guarded ceremonially by the Yeoman Warders. Windsor Castle is another royal castle, while Buckingham Palace is primarily a royal residence.