WordNet sense 1 (able to withstand attack):

impregnable, secure, strong, unattackable

WordNet sense 2 (immune to attack; impregnable):

invulnerable

WordNet sense 3 (impossible to assail):

untouchable

WordNet sense 4 (incapable of being transgressed or dishonored):

inviolable

WordNet sense 5 (without flaws or loopholes):

bulletproof, unshakable, watertight

WordNet sense 6 (incapable of being contested or disputed):

incontestable, incontestible