銅牆鐵壁

Simplified: 铜墙铁壁

tóngqiángtiěbì ㄊㄨㄥˊ ㄑㄧㄤˊ ㄊㄧㄝˇ ㄅㄧˋ

    • copper wall, iron bastion (idiom) impenetrable defense

Character by character

Sources