— air-tight
— airtight
— bombproof
— defendable
— defensible
— entrenched
— impalpable
— implacable
— implausible
— impregnable
— inapplicable
— infallible
— infiltrate
— infiltrator
— inflame
— inflammable
— inflate
— inflexible
— influence
— influential
— invalidate
— invaluable
— inviolable
— inviolate
— involuntary
— involve
— invulnerability
— protected
— safe
— secure
— shellproof
— sheltered
— strong
— tight
— unassailable
— unattackable
— unconquerable
— untouchable