A defensive fortification built in opposition to or to balance another fortification; a counter-fortification.
From counter- + sconce (from Dutch schans, meaning a fortification or shelter). A military architectural term for competing defensive structures.
Medieval and Renaissance fortifications used countersconces strategically—if an enemy built a sconce (small fort) to attack your main fortress, you'd build a countersconce to neutralize their advantage. It's architectural chess at the scale of kingdoms.
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