an impregnable fortress or stronghold; describes a city with defenses that are extremely solid and impossible to breach

(literary) an impregnable fortress; literally a moat of boiling water and a wall of metal; describes a city that is extremely well-defended and impossible to break through

a city that is extremely well-defended or fortified; figuratively used to describe a position or argument that is impregnable or solid

(idiom) a city with walls as hard as stone and a moat of boiling water; used to describe a place that is heavily defended and impossible to breach