Immune to attack; incapable of being tampered with:
— An impregnable fortress.

synonyms: inviolable, secure, strong, unassailable, unattackable.

Roget 159: strong, mighty, vigorous, forcible, hard, adamantine, stout, robust, sturdy, hardy, powerful, potent, puissant, valid.    resistless, irresistible, invincible, proof against, impregnable, unconquerable, indomitable, dominating, inextinguishable, unquenchable; incontestable; more than a match for; overpowering, overwhelming; all powerful, all sufficient; sovereign.    able-bodied; athletic; Herculean, Cyclopean, Atlantean^†; muscular, brawny, wiry, well-knit, broad-shouldered, sinewy, strapping, stalwart, gigantic.    manly, man-like, manful; masculine, male, virile.    unweakened^†, unallayed, unwithered^†, unshaken, unworn, unexhausted^†; in full force, in full swing; in the plenitude of power.    stubborn, thick-ribbed, made of iron, deep-rooted; strong as a lion, strong as a horse, strong as an ox, strong as brandy; sound as a roach; in fine feather, in high feather; built like a brick shithouse; like a giant refreshed.    ... show more

Roget 664: safe, secure, sure; in safety, in security; on the safe side; under the shield of, under the shade of, under the wing of, under the shadow of one's wing; under cover, under lock and key; out of danger, out of the woods, out of the meshes, out of harm's way; unharmed, unscathed; on sure ground, at anchor, high and dry, above water; unthreatened^†, unmolested; protected etc. v.; cavendo tutus [Lat.]; panoplied etc. (defended) 717^†.    snug, seaworthy; weatherproof, waterproof, fireproof.    defensible, tenable, proof against, invulnerable; unassailable, unattackable, impenetrable; impregnable, imperdible^†; inexpugnable; Achillean^†.    safe and sound etc. (preserved) 670; scathless etc. (perfect) 650; unhazarded^†; not dangerous etc. 665.    unthreatening, harmless; friendly (cooperative) 709.    protecting, protective etc. v.; guardian, tutelary; preservative etc. 670; trustworthy etc. 939.    ... show more

Capable of conceiving.

synonym: conceptive.

Incapable of being overcome, challenged or refuted:
— An impregnable argument.
— Impregnable self-confidence.

synonym: inexpugnable.