impregnable (adj.)
- deathless
- defended
- guarded
- immortal
- impenetrable
- imperishable
- incontestable
- incorruptible
- indelible
- indestructible
- indomitable
- ineradicable
- inexpugnable
- inextinguishable
- insuperable
- invincible
- inviolable
- invulnerable
- irresistible
- mighty
- overpowering
- overwhelming
- protected
- quenchless
- resistless
- safe
- secure
- shielded
- unassailable
- unbeatable
- unconquerable
- undestroyable
- undying
- unquenchable
- unsubduable
- unsurmountable
- unyielding
