impregnable Synonyms

strong inexpugnable secure unassailable inviolable unattackable conceptive steady steadfast stable sound safe solid shielded rigid robust well defended stiff substantial well fortified undefeatable unbeatable watertight bulletproof unyielding viable unshakable unconquerable tight tenable resistant protected proof impassable immune hard fortified firm FAST extinguishable durable dreamless defensible defendable constant impenetrable imperdible permanent lasting invulnerable