Tom Shippey points out in his Road to Middle Earth that the germ of Barad Dur, Sauron's Stronghold, comes from a scrap of Chaucer where the poet makes an offhand reference to a knight and his approach to "the dark tower." Chaucer expected that everyone knew that story, but somehow in the intervening centuries it has become lost. Using his imagination, Tolkien tried to delve back into the mine of story and imagine what this Dark Tower might have been. We see several tries at this image, or several "accounts" in Tolkien's corpus. The first is Thangorodrim, Morgoth's "dark tower," where he sits "on hate enthroned." The second, and like unto it, is Sauron's original keep at Tol Sirion. This is the dark tower before which Luthien, in all her frailty, stands and lays the deepest pits bare with her song (an image oddly reminiscent of protestant poets like Spenser, Bunyan, and Wesley). Building on these two images, Tolkien constructs his final Dark Tower, Barad Dur, the body-fortress of Sauron. This is the only dark tower that was known to the public in Tolkien's lifetime. The other two would have been lost in the mines of story had not Christopher Tolkien gone dug into his father's papers and brought them to light in the published Silmarillion. Interestingly, however, after all that work to bring the elusive "dark tower" to light, Tolkien deliberately begins the process of mythologizing it at the end of his Return of the King. At Aragorn's coronation we find Ioreth telling her kinswoman that a halfling and his squire went deep into the enemy's territory and set fire to his tower. Right after the events have happened they are already being obscured and passing into legend. The Dark Tower begins to fade at the very moment of its recovery.
In a literal sense, this is true. Tolkien's recovery of The Dark Tower as a mythopoeic image has led to the obscuring of his particular recovered image in popular culture. In 1977, Terry Brooks gave his Warlock Lord his own re-imagined "dark tower." Stephen King has his "Dark Tower" as well. In how many other ways has this symbol been reworked and re-imagined since Tolkien brought it to light in the 1950s? Even the films by Peter Jackson, due to the unique requirements of story-telling via film, have exerted a subtle distorting effect on Tolkien's original Barad Dur.
What does all this mean? It might mean that symbols and stories are more powerful than the people who create them. It might mean that the most powerful images are not actually created so much as "unearthed," or "reforged." That, of course, begs the question of where such images come from if not ultimately from the mind of a human creator. We could also ask if anyone has the "right" to fix an image, to create the definitive "dark tower." After reading the Iliad, the relationship between recovery and loss, memory and forgetting stands out strongly in my mind. G.K. Chesterton says that human beings are like Robinson Crusoe, survivors of some unaccountable shipwreck. We are forced to work with the fragments that surround us and somehow shore ourselves together. What caused the shipwreck is unknown, but all art and poetry, according to Chesterton, means that for a moment we remember that we have forgotten.