It was a breakaway realm of the Dúnedain kingdom of Arnor. After the death of its tenth King Eärendur, his sons divided the kingdom into the kingdoms of Arthedain, Rhudaur and Cardolan.
Amlaith founded Arthedain and a younger son (Eärendur had many sons so his name or order of birth are unknown) founded Cardolan.
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 (which turned a vassal of Angmar when it appeared).
Eventually around 1409 T.A. the kingdom was destroyed, but its former inhabitants accepted the King of Arthedain as their lord.
The name Cardolan appears to be Sindarin for "Red Hill Land".
Cardolan was a breakaway realm of the Dúnedain kingdom of Arnor.
Cardolan stretched from the river Baranduin (Brandywine) to the Mitheithel (Hoarwell).
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.
Cardolan was a successor realm of the Dúnedain kingdom of Arnor.
In T.A. 1050, the branch of Hobbits known as the Harfoots crossed the Misty Mountains, and settled in the South Downs in the west of Cardolan.
Until the end of the Third Age, the Dúnedain of Cardolan were only a memory, their tombs and barrows haunted by evil wights sent from Angmar; for the Rangers that wandered over the lands were descended from the people of Arthedain.