The Tower of London
Inspiring fear and admiration in equal measure, the Tower of London has a long, grim history that dates back to the Norman conquest in 1066. William the Conqueror founded the fortress after his victory at the Battle of Hastings, to awe and subdue Londoners. And though it was originally used as a royal residence, it's most famous (or infamous) as a prison. King Henry VI and Queen Elizabeth I were both held captive in the Tower; it was the place of execution for unfortunate ladies of the Tudor court like Lady Jane Grey and Henry VIII's queens Anne Boleyn and Catherine Howard; and it was here that Guy Fawkes was tortured before his own execution after he attempted to blow up the Houses of Parliament.
Today you can tour the White Tower, the oldest fortress in the Tower of London, wincing at the sight of the block and axe which are still kept as a sinister reminder of the building's previous purpose. Make sure you take a look at the Traitor's Gate, the infamous water gate through which many famous prisoners passed into their captivity in the Tower. And take a moment during your tour to admire the smart uniforms of the "Beefeaters", who still act as wardens for the Tower, and to meet the resident ravens: legend has it that if they ever fly away, the Tower will crumble and the whole kingdom will fall in ruins.