Same word as in-expugnable:
impregnable, indefeasible, indomitable, inexpugnable, innate, insuperable, intractable, invincible, invulnerable, irrepressible, resistant, secure, unbeatable, unsurpassable, unyielding
impregnable, indefeasible, indomitable, inexpugnable, innate, insuperable, intractable, invincible, invulnerable, irrepressible, resistant, secure, unbeatable, unsurpassable, unyielding