noun
iron wall; impregnable fortress
Literal or figurative strong defensive barrier; often used in military, sports, or security contexts.
敵の鉄壁の守りを破るのは難しい。
It is difficult to break through the enemy's iron wall of defense.
彼のガードは鉄壁だ。
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.
鉄壁の防御を誇るチームだ。
It's a team that boasts an impregnable defense.
その計画は鉄壁のアリバイがあった。
The plan had a cast-iron alibi.
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.