Minas MorgulMinas Morgul - Originally known as Minas Ithil ("Tower of the Moon"), it was built by the Men of Gondor to keep an eye on Mordor.  But early in the Third Age, it was captured by the Enemy and occupied by the Nazgûl.  From that point it became Minas Morgul ("Tower of Sorcery").

 

"Therefore when Osgiliath was taken and the bridge broken Sauron stayed the assault, and the Nazgûl were ordered to begin the search for the Ring.  But Sauron did not underesteem the power and vigilance of the Wise, and the Nazgûl were commanded to act as secretly as they could.  Now at that time the Chieftain of the Ringwraiths dwelt in Minas Morgul with six companions, while the second to the Chief, Kamûl the Shadow of the East, abode in Dol Guldur as Sauron's lieutenant, with one other as his messenger." ~Unfinished Tales (The Hunt for the Ring)