Their chief city was Osgiliath, Citadel of the Stars.
_ _The second part is called_THE TWO TOWERS_, since the events recounted in it are dominated by_ORTHANC_, the citadel of Saruman, and the fortress of_MINAS MORGUL_ that guards the secret entrance to Mordor; it tells of the deeds and perils of all the members of the now sundered fellowship, until the coming of the Great Darkness.
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If the citadel falls, all is lost.
The New York Stock Exchange is a citadel of capitalism.