impregnable castle walls
Literal meaning of the yojijukugo, referring to fortress walls that cannot be breached.
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.
His logic was unassailable, leaving no room for counterargument.
The impregnable defense shut down the opponent's attacks.