During the War of the Ring, there was a fair and green plain called the Pelennor Fields surrounding Gondor's fortress-city of Minas Tirith. Here the crucial Battle of the Pelennor Fields was fought, and the tide of the war turned.

Pelennor means the "fenced land". It was called this because it was encircled by a defensive wall called the Rammas Echor that was built by one of the Ruling Stewards, Ecthelion ll, in the year 2954 of the Third Age.

The Rammas Echor was rapidly breached by the army of the Witch-king when he advanced upon Minas Tirith during the War of the Ring. Fortunately, the Rohirrim cavalry drove the Witch-king's forces onto the fields where eventually his evil hordes were overcome and destroyed.
Encyclopedia entry originally written by Gil_Estel101