Warning: Spoilers follow
After the fall of Osgiliath there was no longer a barrier against the forces of Mordor, which moved on the Pelennor Fields before the city on March 15, 3019 of the Third Age (N. B.: not a date in the Gregorian Calendar) as the Great Darkness blotted out the sun.
Mordor's troops consisted of more than 30,000 Easterlings and Haradrim, numerous oliphaunts, and thousands of Orcs; The defenders' numbers were considerably less despite the addition of about 3,000 men from southern Gondor in the days before the battle.
The attackers used catapults not only to attack the city but also to fire the heads of slain men from Osgiliath into it.
Before dawn the great battering-ram Grond was used to break the city's main gate, and the Witch-king rode into the city unchallenged, save by Gandalf. Before Gandalf's strength was put to the test, however, the cock crowed and the horns of Rohan were heard as around 6,000 of their riders joined the battle. Mordor's strategy for keeping Rohan out of the battle had failed, and the Witch King was forced to ride out and attack them instead of fighting Gandalf.
When the Witch-king's fell beast attacked King Théoden of Rohan, the king's horse Snowmane fell on top of him with fatal results. The warrior Dernhelm, defending the king's body, slew the fell beast and challenged its rider. The Witch-king sneered that no living man might slay him, but the Hobbit Meriadoc Brandybuck wounded him with a sword that had been forged centuries before during the war between Arnor and Angmar and which contained spells against the Witch-king. The spells finally found their target, for the Witch-king became mortal and was slain by Dernhelm, now revealed as Théoden's niece Éowyn and thus no man at all. But the black breath caused both Merry and Éowyn to become gravely ill, and they were sent to the Houses of Healing in the city.
Meanwhile Faramir, son of Denethor, Steward of Gondor, was also gravely wounded. Despairing at the visions of defeat that Sauron had sent him via his Palantír, and believing Faramir to be beyond aid, Denethor prepared to burn himself and his son upon a funeral pyre. Only the intervention of Peregrin Took and Gandalf saved Faramir, but Denethor immolated himself before they could prevent it.
One of the visions that Denethor had seen was of a fleet of enemy ships with black sails arriving at the landings to the south of the Pelennor, but what he had not seen was that they were actually crewed by Aragorn, Legolas and Gimli who had brought an army of the dead - the Oathbreakers of Dunharrow - to fulfil their ancient oath by fighting for Gondor. With their aid the tide of battle was finally turned, and a brief respite was won until the Last Battle before the Black Gate.
The battle is the major centrepiece of Peter Jackson's movie The Lord of the Rings: The Return of the King, although many of the events described above are simplified or altered for cinematic purposes.