The name given to the southerly regions of Arnor, between the rivers Baranduin and Gwathló. Cardolan was a roughly rectangular land, running southwestwards from Amon Sûl to the shores of the Great Sea. Its shoreward parts were known by the name Minhiriath, the land 'between two rivers'.
The name Cardolan seems to have been used from early in Arnor's history, but after the death of KingEärendur of Arnor in III 861, it became a kingdom in its own right. At that time, Eärendur's sons divided the North-kingdom between them, and Cardolan's first king appears to have been Eärendur's second4 son, whose name history does not record.
During the early part of Cardolan's history, it vied with the neighbouring lands of Arthedain and Rhudaur over the control of the Tower of Amon Sûl and its palantír. At this time, at least part of Cardolan's northern border with Arthedain was fortified by a hedged dike and wall, the remains of which survived to the time of the War of the Ring.
After this time, with the loss of its royal lineage, Cardolan was no longer properly a kingdom. Nonetheless a band of Dúnedain endured in the downs and the forest for more than two centuries, until the coming of the Dark Plague. The last of the Dúnedain, and most of the land's other inhabitants, were lost to the Plague, and as a final blow the Witch-king sent evil beings to infest the Barrow-downs. After these events, Cardolan was left a desolate and deserted land.
Together, the Barrow-downs and the Old Forest formed a spur or pocket in the northern borders of Cardolan, creating a small but defensible region apart from the main land. It was here the last Dúnedain of Cardolan settled after the fall of their kingdom. That same area coincides almost exactly with the land under the power of Tom Bombadil at the end of the Third Age.
It seems that Tom was not only present during the chaos surrounding Cardolan's fall, but he knew at least some of its people. After discovering a blue-jewelled brooch from the tomb of the last prince of Cardolan, Tom says:
'Fair was she who long ago wore this on her shoulder. Goldberry shall wear it now, and we will not forget her!'
The Fellowship of the Ring I 8 Fog on the Barrow-downs
He seems to be moved by a recollection of the brooch's wearer, to the extent that he seeks to preserve her memory. That suggests that Tom not only knew the people of Cardolan, but was on friendly terms with them.
This raises interesting questions about the last days of the kingdom of Cardolan. It may be that the Dúnedain retreated into the Barrow-downs not merely as a last line of a defence, but to seek the aid of Bombadil, the powerful spirit that dwelt there. Or it may be that they knew nothing of Tom before they arrived in his Forest, only discovering him there after their arrival.
Did Tom lend the Men of Cardolan any direct aid? That's an impossible question to answer, but if he did, it would help to explain how they survived the assault of an overwhelming enemy force, and how they remained unmolested for the next two hundred years.
This would also help to solve another mystery: if the Witch-king knew where the last Dúnedain of Cardolan were to be found, and had the power to send wights against them at will, why did he wait for two centuries before doing so? If we imagine Tom protecting those within the borders of his lands, this delay makes rather more sense. Only after his charges were devastated by the Plague - so that Tom's protection was no longer needed - were the evil spirits of Angmar able to invade the Barrow-downs.
Manifestly, there's a great deal of supposition here, and this is quite a weight of speculation to build on the only known fact, Tom's recognition of the ancient brooch. However, it does offer a thread of explanation for some of the less accountable events of the war against Angmar.
It is difficult to pin down the end of Cardolan to an exact date, but III 1409 was the year in which Angmar launched its great assault on Amon Sûl, in which Cardolan lost its last prince, and was laid waste by the Witch-king's armies. After that disaster, a remnant of its people survived among the Barrow-downs and in the Old Forest until III 1636, when most of the remaining population was lost to the Great Plague.
Both Tharbad and Lond Daer lay on the River Gwathló, and were thus on Cardolan's southern border, though it is unclear whether Cardolan made any formal claim to either of them. Tharbad was still a living city at this date, though the status of Lond Daer is unclear, and it may already have fallen into ruins; at the very least, the Men of Cardolan would have been a common sight on Tharbad's streets.
The reference to red hills in Cardolan's name probably relates to the red rock that was common in this area (for instance, near the Ford of Rivendell some miles to the east of this region, the East-West Road 'plunged into a deep cutting with steep moist walls of red stone'; The Fellowship of the Ring I 12, Flight to the Ford). Perhaps this rock came to the surface in places as outcrops or tors (most likely among the South Downs) creating the red hills that gave Cardolan its name.
Of the three kingdoms formed by the break-up of Arnor, we know that Arthedain was taken by Eärendur's eldest son, Amlaith of Fornost. The assignment of the other kingdoms is not discussed in detail, but the fact that Cardolan is almost universally listed second of the three strongly suggests that it was the kingdom acquired by the second of Eärendur's sons.