铜墙铁壁（銅牆鐵壁）　

◎ 铜墙铁壁 tóngqiáng-tiěbì

[bastion of iron,impregnable fortress as a wall of bronze;impregnable like walls of brass and iron] 亦作“铁壁铜墙”,比喻防御工事坚固,也比喻力量的强大、意志的坚定