Minas TirithMinas Tirith - Whereas Minas Ithil was built on the east end of Gonder to guard against Mordor, Minas Anor was built on the west end to guard against the Easterlings and other Men who were allied with Sauron.

 

"...and westward at the feet of the White Mountains Minas Anor they made, Tower of the Setting Sun." ~The Fellowship of the Ring, Book II, chap. 2 (The Council of Elrond)

 

But when Mordor captured Minas Ithil and invaded Gondor, the people abandoned Osgiliath and retreated to Minas Anor.  It became known as the White City and was the glory of Gondor.

 

"Then Osgiliath, which in the waning of the people had long been deserted, became a place of ruins and a city of ghosts.  But Minas Anor endured, and it was named anew Minas Tirith, the Tower of Guard." ~The Silmarillion (Of the Rings of Power and the Third Age)

 

"...and the two cites (Minas Tirith and Minas Morgul) were ever at war." ~The Fellowship of the Ring, Book II, chap. 2 (The Council of Elrond)