Synonyms of the word 
INEXPUGNABLE → IMPREGNABLE - UNCONQUERABLE
inexpugnable
- adj. Impossible to eliminate or destroy; impregnable.
impregnable
- adj. (of a fortress, wall, etc., also used figuratively) Too strong to be penetrated.
- adj. Capable of being impregnated.
unconquerable
- adj. Not conquerable; indomitable.
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