Dunharrow was a refuge of the Rohirrim hidden in the White Mountains south of Edoras and fortified against attack. Dunharrow was a high clifftop overlooking Harrowdale, the valley of the river Snowbourn. It was located at the foot of the mountain Starkhorn, on its north side stood the peak of Írensaga, and on the east the great black mountain Dwimorberg.
Dunharrow had been used as a refuge sacred place by the Pre-Númenórean Middle Men of the White Mountains during the Second Age — nearly three millennia before the establishment of the Kingdom of Rohan.
When the Rohirrim came to the region, they recognised the "heathen fane" and they used it as a refuge.
When Rohan was invaded by the Dunlendings in T.A. 2758 some Rohirrim, including Fréaláf, took refuge in Dunharrow during the Long Winter of 2759; Fréaláf led a daring surprise raid from Dunharrow against the Dunlendings in spring, that recaptured Edoras.
Some Orcs who fled the War of the Dwarves and Orcs tried to traverse Rohan and reach the White Mountains. Those invaders were hunted down by Brytta Léofa. However some were seen still lingering in Dunharrow, where King Walda was killed by an Orc in T.A. 2851.
Aragorn, Gimli, Legolas, and the Grey Company arrived at Dunharrow on 7 March T.A. 3019 and then left at dawn the next day to take the Paths of the Dead. Théoden and his riders (with Merry) came to Dunharrow on 9 March and rode out for Gondor on the 10th.
Other versions of the legendarium
Tolkien had made an illustration of Dunharrow and the Firienfeld, that was first published in The Lord of the Rings 1977 Calendar. That illustration represented an earlier conception of the Dark Door, that didn't fit the description in the published text. Christopher Tolkien noted that the Dimholt and the pillar of stone are absent. Tolkien himself had added the note 'No longer fits story'.
- J.R.R. Tolkien, The Lord of the Rings, The Return of the King, "Map of Rohan, Gondor, and Mordor"
- J.R.R. Tolkien, The Lord of the Rings, The Return of the King, "The Muster of Rohan"
- J.R.R. Tolkien, The Lord of the Rings, Appendix A, "The House of Eorl"
- J.R.R. Tolkien, The Lord of the Rings, Appendix B, "The Great Years"
- J.R.R. Tolkien, The Lord of the Rings, Appendix E, "Pronunciation of Words and Names", Note
- J.R.R. Tolkien, "Nomenclature of The Lord of the Rings" in Wayne G. Hammond and Christina Scull (eds), The Lord of the Rings: A Reader's Companion, pp. 769
- J.R.R. Tolkien, Christopher Tolkien (ed.), Pictures by J.R.R. Tolkien, picture 29. Dunharrow
- J.R.R. Tolkien, Christopher Tolkien (ed.), Sauron Defeated, "Part One: The End of the Third Age: Appendix, Drawings of Orthanc and Dunharrow", p. 140 and p. 141, p. 136 and p. 137