The phrase "impregnable to" is correct and usable in written English.
You would typically use this phrase to refer to someone or something that is too strong or resistant to be affected by something. For example, "The team had an impregnable defense to their competitors' best efforts."
On the contrary, it turns American adventurism into an endearing myth that becomes impregnable to protest.
What remains impregnable to them are those refuges that shelter so many men: ordinariness and muddling through.
Troilus and Criseyde by Geoffrey Chaucer Inside the impregnable walls of Troy handsome Troilus fancies himself impregnable to love.
Today's display cases have to be impregnable to thieves but easily accessible to those authorised to handle them.
This referred to the moment when Iran had enough centrifuges in the Fordow facility, which is impregnable to Israeli conventional weapons, to continue enrichment even after an attack.
As December blew into Lithuania, the forest became a fortress of snow, as impregnable to filmmaking as it was to enemy assault 60 years ago.
It can be easy, living within our sophisticated urban sprawl, to feel impregnable, immune to the extreme conditions which all too often buffet the other areas of the globe.
Novak Djokovic was not exactly short of upsides to his game, but the world number one appears to have added an impregnable serve to his armoury.
The tomb is surrounded by jagged mountains that have made Afghanistan such an impregnable fortress to those who have come to fight there, including the United States for the past decade.
The pledge came as the first cracks began to appear in the seemingly impregnable opposition to gun controls.
Searle's brigade reached Singapore in January 1942, just in time to witness the capitulation of the supposedly impregnable fortress to the Japanese.
