Hill in Eriador, also known as Amon Sul. Weathertop was the southernmost of the Weather Hills. It was located just north of the Great East Road east of Bree.
The Tower of Amon Sul was built on Weathertop after the North-kingdom of Arnor was established in 3320 of the Second Age. One of the Palantiri was kept there. After the North-kingdom was divided in 861 of the Third Age, possession of the Weather Hills and particularly the Tower of Amon Sul became a source of dispute among the three kingdoms.
By the end of the Third Age, Weathertop was desolate and uninhabited, except for Rangers who took shelter in the dell on the west side of the hill from time to time. The Ranger known as Strider decided to lead Frodo Baggins and his companions to Weathertop when the Ring-bearer was pursued from Bree by Ringwraiths.
They arrived at Weathertop on October 6, 3018, and set up camp in the dell. In the center of the ruins on top of the hill there was a blackened pile of stones. The topmost stone bore markings that led Strider to believe that Gandalf had been there on October 3. The lights and flashes that Strider and the Hobbits had seen three nights before had in fact been the result of a battle between Gandalf and the Nine Ringwraiths atop Weathertop.
Suddenly on the road below, Frodo saw five of the Ringwraiths. Strider had the Hobbits hide and they retreated to the dell, but the Ringwraiths sensed them and that night they attacked the camp. They were led by the Witch-king of Angmar who was the Lord of the Ringwraiths.
In the presence of Sauron's dreadful servants, Frodo succumbed to the overwhelming temptation to put on the Ring, but he resisted their attempt to take him by drawing his sword and invoking the name of Elbereth Gilthoniel, one of the Valar. The Witch-king stabbed Frodo in the shoulder, but Frodo's defiance and the appearance of Strider armed with flaming brands drove the Ringwraiths away.
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In that time Glorfindel (Arwen in the movie) came to help and to take Frodo safely to Rivendell.
In October of 3018, word came to Rivendell from Gildor Inglorion that Frodo Baggins was heading for Rivendell pursued by the Nazgul led by the Witch-king. Glorfindel was one of the few in Rivendell powerful enough to face the Nine Nazgul. On October 9, he set out on his white horse Asfaloth and followed the Great East Road to the Last Bridge on the River Hoarwell. There on October 11, Glorfindel found three Nazgul waiting and they fled before him. Glorfindel left a green stone on the bridge as a token that it was now safe to cross and he pursued the Nazgul westward. He found two more Nazgul who fled southward from him.
At the Ford of Bruinen on October 20, the Nine Nazgul appeared. Frodo felt compelled to stop, but Glorfindel/Arwen called out to Asfaloth to ride on and the horse carried Frodo across the river. On the far side, Frodo once again felt commanded to halt and he turned to face the Nazgul. Glorfindel/Arwen was aware that Elrond would command the waters of the river to rise against the Nazgul, so he and the others kindled fire to drive the Nazgul into the river. Glorfindel revealed himself as a mighty Elf-lord in all his wrath, and he shone with a brilliant white light as he advanced. The Nazgul were swept away by the floodwaters and returned bodiless to Mordor.