A fortified wall built opposite to or as a defense against another wall, especially one built to strengthen an outer defensive perimeter.
From 'counter-' (opposite, defensive) and 'wall,' from Old English 'weall.' Medieval fortress architecture required multiple concentric counterwall systems.
Medieval castles used counterwall systems so sophisticated they created kill zones where attackers were trapped between multiple walls and could be targeted from above—architectural chess at its deadliest.
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