Minas Tirith is the capital of Gondor since 1636 T.A. when the great plague forced the kings out of Osgiliath. Until 2002 T.A. Minas Tirith was known as Minas Anor. In that year Minas Ithil was captured by the Ringwraiths and renamed Minas Morgul
Minas Tirith means "Tower of Guard", for since the fall of Minas Ithil, Minas Tirith guards Gondor against evil from Mordor.
Minas Tirith is surrounded by the Rammas Echor, a large ringwall encircling the Pelennor Fields. This wall was built by Ecthelion II but proved no match for the Orc legions of Mordor. The city itself lies on a hill beneath Mount Mindolluin. The city is divided into seven one-hundred-foot high layers, each surrounded by white walls. The gates did not lay behind each other but each faced a different direction. Finally within the seventh wall is the Citadel with its White Tower, three hundred feet high, so that its summit is one thousand feet above the plain. Upon the saddle between the city and Mindolluin are the Tombs of the Kings.
In The Return of the King, Minas Tirith is besieged by troops of Mordor, the Easterlings and the Haradrim, under the Great Darkness generated by Mordor. The Battle of the Pelennor Fields takes place on March 15 3019 T. A. Despite heavy losses, the battle is finally won by Gondor.
The eagle who brings the news of Sauron's defeat to Minas Tirith refers to the city as the Tower of Anor. Although Tolkien does not make it clear, it is possible that the city may have reverted to ts original name once it no longer needed to guard against evil.
The Silmarillion reveals that the name Minas Tirith had previously been used in the First Age for Finrod's fortress upon Tol Sirion which guarded the Pass of Sirion. It was later conquered by Sauron, closing the pass. When Finrod and his companions attempted the pass in disguise they were captured and died in the fortress's dungeons. Soon aferward Lúthien and Huan overpowered Sauron and Lúthien destroyed the fortress.
(Some of the above paragraph researched from Robert Foster's Complete Guide to Middle-earth)