impregnable

adjective3

1. •
   Of a fortress or other fortified place: able to withstand all attacks; impenetrable, inconquerable, unvanquishable.

   • Jurgis got up, wild with rage; but the door was shut and the great castle was dark and impregnable. — 1905
2. •
   (figurative) Too strong to be defeated or overcome; invincible.
3. •
   Capable of being impregnated; impregnatable.

   • The reproductive strategies of troop members, especially those of impregnable females, are suggested to influence patterns of range use. — 1979