impregnable fortress; heavily guarded fort
Rare yojijukugo describing a fortress so solidly defended that it cannot be breached. Used in literary or historical contexts.
This castle was truly an impregnable fortress, and the enemy could not easily approach it.
彼の守備は堅塞固塁のごとく、相手チームは一点も取れなかった。
His defense was like an impregnable fortress, and the opposing team couldn't score a single point.
難攻不落 is a common yojijukugo meaning 'impregnable' or 'invincible', used for both literal and figurative strongholds. 堅塞固塁 is a rare synonym with a more classical, literary feel.
A yojijukugo composed of four kanji: 堅 (solid), 塞 (fortress), 固 (firm), and 塁 (rampart). The exact historical derivation is uncertain; it is a rare literary compound.