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Its principal purpose was to defend Ithilien from attacks from Sauron's remaining servants. For that reason its two bastions were directed to the north- and southeast. It also served to stop Sauron's servants from returning to Mordor.
Gondor occupied the fortress probably till T.A. 1636 when the Great Plague killed large parts of Gondor's population. After the Plague Gondor never again occupied Cirith Ungol and evil was allowed to return to Mordor. The fortress was associated with the mountain fortress of Durthang in Northwestern Mordor, and the Towers of the Teeth at the Morannon.
It is not known whether the pass and the tower were called Cirith Ungol when the Men of Gondor held it since Shelob couldn't have inhabited the cavernous entrance of the pass at that time.