The idiom of Iron Wall is similar.

[Pronunciation]

[Explanation] A city made of metal and a moat made of boiling water. Describe the fortifications as extremely strong.

[Source] "Han Kuai Tong Biography": "The land of border towns; Will stick to the baby city; All are Jincheng Tang Chi; Can't attack. "

[example] 1 Under the rapid attack of the German army, this line of defense, which the French army called itself ~), collapsed instantly.

[Proximity] Unbreakable, as steady as Mount Tai, the iron wall of the copper wall will collapse at the touch of a button, Jincheng Tang Chi is rock solid, and An Ru is rock solid.

[antonym] In danger of falling apart