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Cardolan stretched from the river Baranduin to the Mitheithel. Its northern border was the Great East Road, but Cardolan also claimed the Weather Hills, which contained the fortress of Amon Sûl. Amon Sûl housed a palantír, and for this reason it was also claimed by Rhudaur. This lead to a bitter conflict which continued until Rhudaur became a vassal of Angmar after the line of the Dúnedain failed there.
When the kingdom of Angmar appeared in northern Eriador, Cardolan became the most important ally of Arthedain. It had to fight the combined armies of both Angmar and Rhudaur. It soon became apparent that Cardolan could not hold back the forces of Angmar, and in its last years the people became entrenched in their core region, Tyrn Gorthad. In TA 1636 those people who remained in the Barrow-downs died from the Great Plague.
Angmar then sent Barrow-wights to infest and haunt the downs. Arthedain managed to reconquer the land briefly, but few people wished to live there on account of the Barrow-wights, and Cardolan was soon lost again.
After the final fall of Arnor, and the destruction of Angmar at the Battle of Fornost, Cardolan remained an unpopulated area until the reestablishment of the northern kingdom under king Elessar at the end of the Third Age.
It is not known if Tolkien ever explained the name Cardolan. The most common suggestion is that Cardolan likely is Sindarin for "red hill country". In that case, the name could be analyzed as carn "red", dol "hill, mount" and an(n) "land".
An alternative etymology has been suggested by Roger Clewley: Cardolan deriving from Noldorin car "house", dolen "hidden, secret", and the toponymical ending -and, thus meaning "place/land of hidden houses" (a reference to the "dead entombed there").