"In that dreadful light Sam stood aghast, for now, looking to his left, he could see the Tower of Cirith Ungol in all its strength. The horn that he had seen from the other side was only its topmost turret. Its eastern face stood up in three great tiers from a shelf in the mountain-wall far below; its back was to a great cliff behind, from which it jutted out in pointed bastions, one above the other, diminishing as they rose, with sheer sides of cunning masonry that looked north-east and south-east. ... As he gazed at it suddenly Sam understood, almost with a shock, that this stronghold had been built not to keep enemies out of Mordor, but to keep them in. It was indeed one of the works of Gondor long ago, an eastern outpost of the defenses of Ithilien, made when, after the Last Alliance, Men of Westernesse kept watch on the evil land of Sauron where his creatures still lurked. But as with Narchost and Carchost, the Towers of the Teeth, so here too the vigilance had failed, and treachery had yielded up the Tower to the Lord of the Ringwraiths, and now for long years it had been held by evil things. "
—The Return of the King
Cirith Ungol was once a tower of Gondor used to watch over Mordor. As Gondor's forces decreased, it was abandoned, and Sauron took it for his own.
In the Edain Mod, Cirith Ungol is a setttlement building for Mordor. From here, the elite orc and uruk units can be built. These units all cost 0 command points but are limited to three battalions each.