Imagine you live in a medieval kingdom, and you're the poor sap the king just named to design his new castle.
"A nice one," he says, "and make sure it's impenetrable!"
(Stick with us here, this'll get relevant soon.)
You might build walls so tall and thick that no machine can penetrate them, stock the mote with alligators, and keep the oil boiling for any barbarian hordes passing by. Keeping people out of the castle isn't hard ... until the king mentions he's after a drawbridge, too.
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