iron wall; impregnable fortress
Literal or figurative strong defensive barrier; often used in military, sports, or security contexts.
It is difficult to break through the enemy's iron wall of defense.
His guard is an impregnable fortress.
noun which may take the genitive case particle 'no'
impregnable; invulnerable; unassailable
Adjectival use with の, describing something extremely solid, unbreakable, or impossible to defeat.
It's a team that boasts an impregnable defense.
The plan had a cast-iron alibi.
impregnable castle walls · impregnable; invulnerable; unassailable
Compound of 鉄 (iron) and 壁 (wall), literally 'iron wall'. The figurative sense of 'impregnable' developed naturally from the image of an unbreakable metal barrier.