impregnable

1 incapable of being defeated, overcome, or subdued an impregnable fortress that had foiled one invader after another over the centuries Synonyms bulletproof, invincible, indomitable, insuperable, insurmountable, invulnerable, unbeatable, unconquerable, unstoppableRelated Words inviolable, unassailable, unbreachable, untouchable; armored, defended, guarded, protected, safe, safeguarded, secure, shielded; unbeaten, unbowed, unconquered, undefeated, unsubduedNear Antonyms exposed, imperiled (or imperilled), insecure, liable, open, susceptible, unguarded, unprotected, unsafe; defenseless, helpless, powerless, weakAntonyms superable, surmountable, vincible, vulnerable

2 impossible to get through or into the castle's supposedly impregnable walls Synonyms impassable (also impassible), impermeable, impervious, impenetrableRelated Words close, compact, dense, thick, tight; compressed, condensed; sturdy, substantial, tough; firm, frozen, hard, solid, stiff; inflexible, rigid, unbending, unyieldingNear Antonyms soft, squishy; bendable, elastic, flexible, giving, malleable, pliable, yielding; absorbent, porousAntonyms negotiable, passable, penetrable, permeable