Weathertop (Amon Sûl in Sindarin) was a large hill in Eriador which had built upon it a formidable watchtower. The watchtower was built by the Dúnedain under Elendil to protect Arnor and the Great East Road. Inside of it was put one of the
Palantíri which made the watchtower even more strategically important as a center of reconnaissance and communication. After the death of the final king of Arnor, King Eärendur, the Kingdom was split and Weathertop lay on the very border between the new realms Arthedain, Rhuduar, and Cardolan. The bickering lords of the crumbling Arnorian kingdoms were eventually defeated by the Witch King of Angmar. He manipulated the hillmen of Rhuduar and brought his orcs from the north, overthrowing the old kingdom. Amon Sûl was sacked by the Witch King and King Arveleg I of Arthedain was slain here. The Dúnedain managed to escape with the Palantír. After it’s destruction it remained a ruin for the remainder of it’s history. During the War of the Ring the watchtower saw two major events occur. Gandalf fought with the Nazgûl on it’s summit, and days later in a different engagement Aragorn defended the hobbits from the Black Riders, but Frodo was stabbed with a Morgul Blade and was gravely wounded.
“The Men of the West did not live here; though in their latter days they defended the hills for a while against the evil that came out of Angmar. This path was made to serve the forts along the walls. But long before, in the first days of North Kingdom, they built the great watch-tower on Weathertop. Amon Sûl they called it. It was burned and broken, and nothing remains of it now but a tumbled ring, like a rough crown on the old hill’s head. Yet once it was tall and fair. It is told that Elendil stood there watching for the coming of Gil-galad out of the West, in the days of the Last Alliance.” -Aragorn explaining the importance of Weathertop to the hobbits. A Knife In the Dark, Fellowship of the Ring.