What is the Tower of London?

There's a castle in London that's almost a thousand years old, sitting right beside the River Thames like it owns the place. It's called the Tower of London, and it's been a palace, a prison, a zoo, a treasure vault, and a place where kings kept their heads — until they lost them.
William the Conqueror started building it in 1066, right after he invaded England and decided he needed a castle to remind everyone who was boss. The White Tower in the middle — the big square keep — was his "I'm in charge now" statement, made of stone so thick you could drive a car through the walls. Well, if cars had existed back then.
