Impregnable
[adj] impossible to take by storm
[adj] able to withstand attack; "an impregnable fortress"; "fortifications that made the frontier inviolable"
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deathless, defended, guarded, immortal, impenetrable, imperishable, incontestable, incorruptible, indelible, indestructible, indomitable, ineffaceable, ineradicable, inexpugnable, inextinguishable, insuperable, invincible, inviolable, invulnerable, irresi