Barad-dûr (Sindarin "Dark Tower", sometimes given as The Barad-dûr, Lugbúrz in Black Speech) is the fortress of Sauron in the fantasy world of J. R. R. Tolkien's The Lord of the Rings. The Eye of Sauron kept watch over Middle-earth from its highest tower.
Barad-dûr was built by Sauron, the Dark Lord of Mordor, with the power of the One Ring, during the Second Age. The building took six hundred years to complete; it was the greatest fortress ever built since the Fall of Angband, and much of Sauron's personal power went into it.
Barad-dûr was besieged for seven years by the Last Alliance of Elves and Men and was levelled after Sauron's defeat at the end of the Second Age, but because it was created using the power of the One Ring, its foundations could not be destroyed completely unless the Ring itself was destroyed. Isildur cut the Ring from Sauron's hand but refused to destroy it, so the Tower was quickly re-built when Sauron returned to Mordor thousands of years later.
An interesting contradiction exists regarding the Dark Tower: Appendix B has Sauron beginning to build the Tower in c.1000 S.A., completing it at the same time he forges the One Ring c.1600 S.A., yet Elrond asserts that its foundations "were made with the power of the Ring." No explanation is offered in the text, but it appears that without the Ring's sustaining power the Tower's foundations provided insufficient support for the structure. Thus when the Ring was destroyed the Tower collapsed. (It might mean that Sauron put his power used to build the foundations of Barad-dûr into the One Ring to keep Barad-dûr standing if he was defeated later. Or conversely, that he used the Ring to invest the foundations with his power, possibly his intent all along, even if the structure had been finished before c.1600 S.A., the Dark Tower was only "complete" (according to its creator's purpose) when the Ring had invested its power into the foundation. Since the Tower was staggeringly huge it might be imagined that some power was required to support it in the physical world, though this is conjecture.) This contradiction has led some fans to joke that the Dark Tower must be pre-fabricated modular housing.
The Dark Tower was described as existing on a massive scale so large it was almost surreal, although Tolkien does not provide much detail beyond its size and immense strength. Since it had a "topmost tower" it presumably had multiple towers. It is otherwise described as dark and surrounded in shadow, so that it could not be clearly seen.
In his vision at Amon Hen, Frodo Baggins perceived the immense tower as "wall upon wall, battlement upon battlement, black, immeasurably strong, mountain of iron, gate of steel, tower of adamant...Barad-dûr, Fortress of Sauron".
There was an "immeasurably high" look-out post, "the Window of the Eye in Sauron’s shadow-mantled fortress", said to face Mount Doom. In this window, Frodo and Sam had a terrible glimpse of the Eye of Sauron.
In the Lord of the Rings film trilogy by Peter Jackson, Richard Taylor and his design team built a 9 meter high miniature ("big-ature") of Barad-dûr for use in the film. Using the size scale for the model implemented for the films, the Dark Tower is depicted as being over 914 m (3,000 feet) tall.
The Lord of the Rings: The Return of the King also shows Barad-dûr as clearly visible from the Black Gate of Mordor. Even granting its enormous size, it was located one hundred miles away and to the east of the Gate, not to mention being behind the inner mountain ridges of Udûn so Aragorn's army would not have been able to see it. It is also shown in front of Mount Doom, but when looking from the gate as shown in the maps of Middle-earth, Barad-dûr is actually somewhat behind Mount Doom. In the film version, the geography of Mordor and Middle-earth in general seems to have been compressed somewhat, perhaps for artistic reasons related to rendering such complex stories in a visual medium. In the case of the Black Gate scene, having Barad-dûr visible from the Gate means that the army can see the Eye of Sauron staring at them. The reason this was done is due to the deleted "Aragorn Vs Sauron" scene. Originally there would be a "blinding light" and Aragorn would see Annatar, who would then become Sauron and attack. However the filmmakers decided this deviated too far from the books, so instead the blinding light scenes were used to depict a "staring contest" between Aragorn and the Eye of Sauron.
Again another deleted scene in the extended edition of ROTK appears to reinforce this view, showing Sauron standing atop his tower and briefly being observed by Aragorn.