The son of Argeleb, King Arveleg I continued his fathers defence against Angmar along the fortified Weather Hills, the Great East Road, and the lower Hoarwell, and for many years he held back the assaults of Angmar and Rhudaur. It was also at this time that the Elven haven of Rivendell was besieged by the forces of Angmar. Arnor's capital of Fornost lay upon the North Downs only a few dozen miles away from the Weather Hills, and Angmar sought a way to break through the frontier and lay siege to the city. It was to this end that in the year TA 1409 a great host came out of Angmar, and crossing the river it entered Cardolan and surrounded Amon Sûl. After a fierce siege the Dúnedain of Arnor were defeated, the tower of Amon Sûl destroyed, and King Arveleg slain. However, the Witch-king's victory was not complete, as the palantír of Amon Sûl, the greatest of the three in the north, was carried back in retreat to Fornost.
After the destruction of Amon Sûl, the power of Arnor to resist the army of Angmar was severely weakened. The tower's destruction meant that the army of Angmar had now made the fortified line along the Weather Hills untenable, and its forces gained easy access to the heart of Arnor. Rhudaur was now absorbed by Angmar and populated by evil men subject to the Witch-King. The few Dúnedain who remained were either slain or fled far westwards, never returning to the lands now claimed by Angmar. The kingdom of Cardolan was ravaged by the forces of Angmar who entered its northern borders uncontested by the crippled army of Arthedain, and its people fled into hiding on the ancient Barrow Downs, or Tyrn Gorthad in Sindarin. It was in this desperate defence against Angmar that the last prince of Cardolan fell in battle, and he was entombed by the survivors among his people in what some say was the very same barrow that Frodo and his companions had been imprisoned during the War of the Ring.Although the situation was dire, the young King Araphor, son of Arveleg, defended the city of Fornost against the host of Angmar. Although he was still a youth by the reckoning of his people, Araphor proved valiant, and with the help of Círdan of Lindon he repulsed Angmar's army from the North Downs and won a great victory. It was at this time that Angmar was subdued by Elven-folk from Lindon, Rivendell, and even Lothlórien for Elrond had brought help over the passes of the Misty Mountains. At the same time, the Stoors (one of the three peoples who were named Hobbits) who had previously dwelt in the angle between the Hoarwell and the Loudwater fled south-westwards because of the wars and the dread of Angmar, and because the land and clime of eastern Eriador worsened and became unfriendly. Some of these returned to Wilderland in Rhovanion and dwelt alongside the banks of the Gladden, becoming a riverside people of fishers, but others migrated westwards to a land known by the Dúnedain as Sûza, which the Stoors named the Shire. These peoples were granted by the eleventh king of Arthedain, Argeleb II, land that had become sparsely populated by war, and dwelling there as a people became the Hobbits known by the end of the Third Age for their love of peace and domesticity.
Amon Sûl was the main defensive outpost in the Weather Hills and northern Cardolan. With it gone the Witch-King was able to take and ravage Cardolan, as well as consolidate his new conquests in Rhudaur, which his Hill-Men allies and subjects had taken, as well as launch attacks against the whole of Arthedain.
However despite permanently ending the threat that was Cardolan, Angmar was unable to ride its momentum, and was driven away from the city of Fornost and the North Downs by the forces of the new King Araphor and Lord Círdan of Lindon.