- "Then at last his gaze was held: wall upon wall, battlement upon battlement, black tower of adamant, he saw it: Barad-dûr, Fortress of Sauron. All hope left him."
- —About Frodo seeing from Amon Hen, in The Fellowship of the Ring
It was the tallest fortress ever built in Middle-earth, standing over 1400 meters in height from top to bottom.
Barad-dûr was built by Sauron in the land of Mordor, not far from the volcano called Mount Doom (Orodruin). The construction of the tower begun around the year SA 1000, and took six hundred years to complete. It was the greatest fortress ever built since the War of Wrath and the fall of Angband, and much of Sauron's personal power went into it.
Barad-dûr was under siege for seven years by the Last Alliance of Elves and Men, and was leveled 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 should be destroyed. Isildur failed to destroy the Ring, and so the tower was re-built when Sauron returned to Mordor thousands of years later.
Some time near the end of the Third Age (TA 3009 - TA 3017), Gollum was captured and taken to Barad-dûr, while Gandalf and Aragorn were also searching for him. He was tortured, and thus Sauron learned that the One Ring had been found, and heard the words Shire and Baggins, before releasing Gollum in TA 3017.
Only when Frodo Baggins (and Gollum) destroyed the One Ring, the Tower was finally brought down. Without Sauron's power from the ring to sustain it, it could not stand. Barad-dûr collapsed to ruin, from base up, and Sauron was finally defeated.
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" (the location of the Window of the Eye, from which the Eye of Sauron gazed out over Middle-earth), it presumably had multiple towers. It is otherwise described as dark and surrounded in shadow, so that it could not be clearly seen. It was known to have giant caverns or "Pits" under the immense structure which could have been prisons.
In the Black Speech, it is called Lugbúrz, meaning "dark tower".
Portrayal in AdaptationsEditIn the Lord of the Rings movies by Peter Jackson, Richard Taylor and his design team built a nine foot high miniature bigature 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 1,500 meters (5,000 feet) tall. 
The Return of the King film 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 probably 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. Also, it is significantly closer to Mount Doom in the Return of the King film than in the previous two films. 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.
Sauron's sanctum is below the apex where the Eye was, and his throne is guarded by the Elite Black Uruks.
Places of Middle-earth and Arda
Forests & Mountains:
The rest of Arda: