During the Middle Ages, it was one of the most impregnable castles in Italy.

That's where all our faith is placed, our most impregnable fortress, as secure as e-alpha.

Few other men, he thought, perhaps none, could have penetrated safely to the very threshold of this most impregnable of all places in the Solar System.

By their very nature, even the most impregnable vault or safe eventually needs to be able to be opened and closed by someone.

Jalore Fort is regarded as one of the most impregnable forts of the country.

A mind that has nothing of hope, or fear, or aversion, or desire, to weaken and disturb it, is the most impregnable security.

At the gates of his own palace, in the heart of the most impregnable Arya city ever built.

Here the Germans have the most impregnable defensive positions in the history of modern warfare.

The thorn boma round it was the strongest and most impregnable that the Mosula could construct.

It was he who made Mandu one of the most impregnable forts of India.