In military science, a line of fortifications built by a besieging army opposite a fortress, facing inward to prevent the besieged from breaking out.
From contra- (against) plus vallation (from Latin vallum 'rampart, wall'). This military term describes fortifications built against an enemy's fortifications during a siege.
Medieval and ancient armies had a brilliant defensive strategy: they'd build their own walls facing *into* the fortress they were attacking, trapping the defenders—basically turning the fortress into a prison! It's siege warfare chess.
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