1) Air-tight 2) Bombproof 3) Defendable 4) Defensible 5) Entrenched 6) French word used in English 7) Immune to attack 8) Impregnable 9) Inviolable 10) Protected 11) Safe 12) Secure 13) Shellproof 14) Strong 15) Tight 16) Unassailable 17) Unattackable 18) Unconquerable 19) Untouchable 1) Airtight 2) Bombproof 3) Defendable 4) Defensible 5) Entrenched 6) Impregnable 7) Protected 8) Safe 9) Shellproof 10) Unassailable 11) Unattackable 12) Unconquerable 13) Untouchable 14) Unvulnerable 
• (a.) Unanswerable; irrefutable; that can not be refuted or convinced; as, an invulnerable argument. • (a.) Incapable of being wounded, or of receiving injury.
In·vul'ner·a·ble adjective [ Latin invulnerabilis : confer French invulnérable . See In- not, and Vulnerable .] 1. Incapable of being wounded, or of receiving injury. « Neither vainly hope To be invulnerable in those bright arms.» Milto...
[adj] - immune to attack
adjective immune to attack; impregnable; `gunners raked the beach from invulnerable positions on the cliffs`
incapable of being wounded, hurt, or damaged. · proof against or immune to attack: A strong navy made Great Britain invulnerable. · not open to denial or disproof: an invulnerable argument.