銅牆鐵壁

tóngqiángtiěbì

a bastion of iron—an impregnable fortress

* * *

tong2 qiang2 tie3 bi4

copper wall, iron bastion (idiom); impenetrable defense

* * *

銅牆鐵壁|铜墙铁壁 [tóng qiáng tiě bì] ► copper wall, iron bastion (idiom); impenetrable defense