The brain is at least twenty feet from his apparent forehead in life; it is hidden away behind its vast outworks, like the innermost citadel within the amplified fortifications of Quebec.
And as upon the invasion of their valleys, the frosty Swiss have retreated to their mountains; so, hunted from the savannas and glades of the middle seas, the whale-bone whales can at last resort to their Polar citadels, and diving under the ultimate glassy barriers and walls there, come up among icy fields and floes; and in a charmed circle of everlasting December, bid defiance to all pursuit from man.
There are no more uses of "citadel" in the book.
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If the citadel falls, all is lost.
The New York Stock Exchange is a citadel of capitalism.