noun, noun which may take the genitive case particle 'no'
Describes a fortress or defensive position that is naturally strong and difficult to attack. Often used in historical or military contexts.
This castle was impregnable, and the enemy could not easily attack it.
要害堅固な地形を利用して砦を築いた。
They built a fort taking advantage of the unassailable terrain.
難攻不落 also means 'impregnable', but is more commonly used and can apply to abstract things like arguments or records, while 要害堅固 specifically emphasizes natural defensive strength of a location.
A yojijukugo composed of 要害 (key strategic point) and 堅固 (firm, solid). The exact historical derivation is uncertain, but the phrase is conventionally associated with describing strong natural fortifications.