The tower of Barad-dûr is in the land of Mordor, the seat of Sauron's power. Sauron is the Lord of the Rings, creator of the One Ring, cut from his hand by Prince Isildur. The Dark Tower, Barad-dûr was abandoned but because it was created using the power of the One Ring it 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.
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Only when Frodo's quest to destroy the One Ring was fulfilled did the Tower finally fall.