impregnable
/ɪmˈpɹɛɡ.nəˌb(ə)l/ adjective C2- Capable of being impregnated; impregnatable.
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.
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