铁壁铜墙

tiěbìtóngqiáng

铁壁铜墙 — tiěbìtóngqiáng

1. an impregnable defence; iron wall and bronze ramparts (used figuratively for an extremely strong fortification or barrier)

   这座古城的守备固若金汤，真可谓铁壁铜墙。 zhè zuò gǔchéng de shǒubèi gùruòjīntāng zhēn kěwèi tiěbìtóngqiáng — The garrison of this ancient city is rock-solid; it can truly be called an impregnable fortress.