Wikipedia
Barad-dûr, the “Dark Tower,” is a fictional place in J. R. R. Tolkien's Middle-earth writings and is described in The Lord of the Rings, The Silmarillion, and other works. It is an enormous fortress of the Dark Lord Sauron, whence he rules the volcanic and barren land of Mordor. Located in northwest Mordor, near Mount Doom, the Eye of Sauron keeps watch over Middle-earth from its highest tower. The name is pronounced "Ba'rad doorr" with emphasis placed on the "rr." The Lieutenant of Barad-dûr is the Mouth of Sauron, who acts as an ambassador and herald for Mordor and Sauron.
Barad-dûr was completed by Sauron in 1600 of the Second Age. It was destroyed following the victory of the Last Alliance of Elves and Men at the end of the age. Rebuilding began in the Third Age, 67 years before the War of the Ring. As the Dark Tower is held together by Sauron's magic, it collapses upon the destruction of the One Ring.