Its walls, roof lines and smokestacks looked trim, impregnable like a fortress.
She looked at the quiet, impregnable room, and at the light-the light that came from his motor-on the faces of men who were the most serene and confident gathering she had ever attended.
The door was not locked, thought Dagny; she felt an unreasoning desire to tear it open and walk in-it was only a few wooden boards with a brass knob, it would require only a small muscular contraction of her arm-but she looked away, knowing that the power of a civilized order and of Ken Danagger’s right was more impregnable a barrier than any lock.
If she left the valley, the screen would close for her as tightly, Atlantis would descend under a vault of rays more impregnable than the bottom of the ocean, and she, too, would be left to struggle for the things she had not known how to see, she, too, would be left to fight a mirage of primordial savagery, while the reality of all that she desired would never come again within her reach, But the pull of the outer world, the pull that drew her to follow the plane, was not the image of…
There are no more uses of "impregnable" in the book.