Why Do Castles Have Portcullises?

Remember the metallic gate with sharp pointed edges at Castle Black of Game of Thrones? They are called Portcullises, and it appears that they do not only have theatrical importance; most castles of medieval times had Portcullises. So the question is this: why do Castles have portcullises, and what is the history behind them?  Portcullises … Read more