Amon SûlAmon Sûl - The southern watchtower for the Kingdom of Arnor.  Built primarily as a defense against a possible invasion from Mordor, it was destroyed when the region was conquered by Angmar.  At the end of the Third Age, it was the location where the hobbits took refuge, but were eventually attacked by the Ringwraiths.

 

"'No.  There is no barrow on Weathertop, nor on any of these hills,' answered Strider.  '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 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 an 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 I, chap. 11 (A Knife in the Dark)