Tāmen de fángshǒu tóngqiáng-tiěbì, duìshǒu nányǐ tūpò.
Their defense was impenetrable; opponents could not break through.
Tips
history
Attested in the Yuan play 《谢金梧》: '随他铜墙铁壁，也不怕不拆倒了他的' - 'however tough his copper walls and iron ramparts, we'll still bring them down'. Yuan drama cemented it as stock military vocabulary.
usage
Figurative use is now more common than literal - sports defenses, cybersecurity, regulatory barriers, tight-knit groups. Often in 形成 / 筑起 + 铜墙铁壁.
