Now she was eagerly curious to know what had decided the Beauforts to invite (for the first time) Mrs. Lemuel Struthers, the widow of Struthers’s Shoe-polish, who had returned the previous year from a long initiatory sojourn in Europe to lay siege to the tight little citadel of New York.
There was always a traitor in the citadel; and after he (or generally she) had surrendered the keys, what was the use of pretending that it was impregnable?
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.