impregnable Synonyms

conceptive strong inviolable inexpugnable unassailable unattackable secure well fortified well defended stiff steady steadfast stable sound solid shielded substantial tenable watertight unbeatable bulletproof unyielding viable unshakable unconquerable undefeatable tight water-resistant safe indestructible impervious impenetrable impassable immune hard firm FAST durable defensible indomitable ineffaceable insuperable robust rigid resistant protected proof permanent lasting invulnerable