noun
impregnable castle walls
Literal meaning of the yojijukugo, referring to fortress walls that cannot be breached.
金城鉄壁の城は、どんな攻撃にも耐えた。
The castle with impregnable walls withstood any attack.
noun which may take the genitive case particle 'no'
impregnable; invulnerable; unassailable
Figurative use describing something extremely secure or impossible to defeat, such as a defense, argument, or system.
彼の論理は金城鉄壁で、反論の余地がなかった。
His logic was unassailable, leaving no room for counterargument.
金城鉄壁の守備陣が相手の攻撃を封じた。
The impregnable defense shut down the opponent's attacks.