Impregnable

Adjective

 * 1) (Of a fortress, wall, etc.) Too strong to be penetrated.

Thesaurus
deathless, defended, guarded, immortal, impenetrable, imperishable, incontestable, incorruptible, indelible, indestructible, indomitable, ineffaceable, ineradicable, inexpugnable, inextinguishable, insuperable, invincible, inviolable, invulnerable, irresistible, mighty, nonperishable, overpowering, overwhelming, protected, quenchless, resistless, safe, safeguarded, secure, shielded, unassailable, unbeatable, unconquerable, undampable, undefeatable, undestroyable, undying, unquenchable, unsubduable, unsurmountable, unyielding

Etymology
Ultimately from im- "not", French prendre "to take", and -able "able to be __-ed".