impregnable
/ɪmˈpɹɛɡ.nəˌb(ə)l/ adjective C2- Of a fortress or other fortified place: able to withstand all attacks; impenetrable, inconquerable, unvanquishable.
- Too strong to be defeated or overcome; invincible.
From the library
He spoke Italian like a Tuscan, and Spanish like a Castilian; he would have been free, and happy with Mercédès and his father, whereas he was now confined in the Château d’If, that impregnable fortress, ignorant of the future destiny of his father and Mercédès; and all this because he had trusted to Villefort’s promise.
Other matches
Learn impregnable in real texts
With a free account, hover any word in the library for its meaning, hear it read aloud, save words for review timed to your memory, and see how much of each book you already know.