The word "impregnableness" refers to the quality of being unconquerable, invulnerable or impenetrable. However, there are several antonyms for this term, which refer to the opposite quality. Terms like penetrability, vulnerability, and conquerability all imply that something is not impregnable, and can be breached, conquered, or penetrated. Other antonyms for impregnableness include weakness, fragility, susceptibility, and accessibility. These words suggest that something is easily penetrable, prone to attack or damage, and can be compromised. Antonyms for impregnableness are useful when you want to highlight the vulnerability, accessibility or penetrability of something, instead of its strength or invincibility.