impregnable, inviolable, secure, strong, unassailable, unattackable -- (immune to attack; incapable of being tampered with; "an impregnable fortress"; "fortifications that made the frontier inviolable"; "a secure telephone connection")
unassailable, untouchable -- (impossible to assail)
unassailable, unshakable, watertight, bulletproof -- (without flaws or loopholes; "an ironclad contract"; "a watertight alibi"; "a bulletproof argument")
(Sound)
