Weathertop

But long before, in the first days of the North Kingdom, they built a 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.
– The Fellowship of the Ring (Book 1), Chapter 11

The southernmost top of the Weather Hills, there had been a tower on Weathertop (or Amon Sûl) dating back to the days of King Elendil. It was here where one of Arnor’s Palantíri was stored for the first half of the Third Age.

The Palantír and its central location made the tower an important strategic position that was sought after by all three successor kingdoms of Arnor, eventually landing in the possession of the kingdom of Arthedain. The Witch King lay siege to Amon Sûl in T.A. 1409 and razed it to the ground upon capturing it. However, a few Dúnedain managed to rescue Amon Sûl’s Palantír and brought it to Fornost Erain.