Castle Highever has stood since the Divine Age, when it was not an independent bannorn, but merely an outpost of the growing Bannorn of Amaranthine, in the days before Amaranthine became an arling itself. The outpost of Highever was originally held by the Elstan family, cousins of the Howes. In the Age of Towers, however, Bann Conobar Elstan was murdered by his wife, Flemeth, thus ending the bloodline. Conobar's captain of the guard, Sarim Cousland, took the lands and title.
The Couslands declared their independence from Amaranthine, starting a war that lasted 30 years. When the dust settled, Highever was on its own, and in possession of half the land that had once been southwestern Amaranthine.
Highever became a teyrnir during the Black Age, when Haelia Cousland gathered the lords together under her banner to drive the werewolves out of their lands, earning herself the title of teyrna almost as an afterthought.
Today, Highever is one of only two remaining teyrnirs, making the Cousland family second in rank only to the king.
If the Warden is Human Noble: The Cousland family, however, was all but wiped out in an unexpected attack by Arl Howe of Amaranthine, and the fate of the teyrnir is now in question.
If the Warden is not Human Noble: Arl Howe of Amaranthine was named the new Teyrn of Highever under somewhat questionable circumstances, and the fate of the Cousland family is now uncertain.
--From Ferelden: Folklore and History, by Sister Petrine, Chantry scholar