adj. 25B6; adjective
an impregnable castle : INVULNERABLE, impenetrable, unassailable, inviolable, secure, strong, well fortified, well defended; invincible, unconquerable, unbeatable, indestructible.
an impregnable parliamentary majority : UNASSAILABLE, unbeatable, undefeatable, unshakeable, invincible, unconquerable, invulnerable.
vulnerable.