Drawbridges were common in medieval times for defense purposes; think of castles. Medieval warriors would defend the fortress by lifting the drawbridge.
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Modern Symbolism
Drawbridges symbolize openness or barriers in stories, sometimes creating tension. In the tale, the lowered drawbridge welcomed all travelers into the kingdom.
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Functional Aspect
In modern use, drawbridges allow traffic flow by alternating between vehicles and boats. The drawbridge schedule caused delays for commuters this morning.