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Tower of London: why the White Tower's front door is a floor up
At the Tower of London, the White Tower's front door is a whole floor up in the air. William the Conqueror built it nearly a thousand years ago. And Londoners did not like him much. So the way in was up a wooden staircase. If an attack came, the guards could take the stairs away. Or simply burn them. And everyone inside was out of reach, behind walls four and a half metres thick. The wooden steps there today copy the kind that stood there first. So stand where you can see those steps, and look how high the door is. Then climb up, and walk in the Norman way.
If you go: Stand where you can see the wooden steps on the White Tower's south side, look up at how high the doorway is, then climb them: you walk in the way the Normans did.
Sources: Historic Royal Palaces · Wikipedia: White Tower (Tower of London)