Synonyms for impregnable

Adjective

1. impregnable, inviolable, secure, strong, unassailable, unattackable, invulnerable (vs. vulnerable)

usage: able to withstand attack; "an impregnable fortress"; "fortifications that made the frontier inviolable"

2. impregnable, inexpugnable, unconquerable (vs. conquerable)

usage: impossible to take by storm