铜墙铁壁

tóngqiángtiěbì

铜墙铁壁 — tóngqiángtiěbì

1. copper wall and iron bastion (idiom); an impenetrable defense

   他们的城防是铜墙铁壁，敌人根本攻不下来。 tāmen de chéngfáng shì tóngqiángtiěbì dírén gēnběn gōngbùxià lái — Their city defenses are like a copper wall and iron bastion; the enemy simply can't capture it.