铜墙铁壁，坚不可摧的堡垒

1. iron wall, impregnable fortress

noun (common) (futsuumeishi)

坚不可摧的，刀枪不入的，无懈可击的，坚如磐石的

2. impregnable, invulnerable, unassailable, cast-iron

nouns which may take the genitive case particle 'no'