J.R.R. Tolkien > The Lord of the Rings >
...wall upon wall, battlement upon battlement, black, immeasurably strong, mountain of iron, gate of steel, tower of adamant... Barad-dûr, fortress of Sauron.
-- J.R.R. Tolkien, The Fellowship of the Ring
Sindarin elvish name literally meaning 'dark tower', Sauron's mighty fortress in the heart of Mordor. Also known in the Black Speech of the orcs as Lugbúrz.
The first Barad-dûr was built in the middle of the Second Age of Arda, when Sauron first came to Mordor, shortly after the Rings of Power were made, and it was in nearby Mount Doom that he forged the One Ring. Soon after, there was war between him and the elves of Eregion, and the Númenorean king Tar-Minastir had sent aid to Gil-galad in this war. However, they were never able to enter Mordor but only prevented Sauron from extending his might west. Many years later, Sauron left the Tower as a hostage of Ar-Pharazôn when he invaded Middle Earth, and his spirit returned to it after the drowning of Númenor. The Last Alliance of Men and Elves besieged the Tower afterwards, and eventually Sauron was defeated, and the Tower was leveled to the ground. Its foundations, however, remained intact, because they were created with the power of the Ring, which Isildur refused to destroy.
This allowed Sauron to rebuild the Tower during the Third Age that followed, and it was only after the One Ring was finally destroyed that the Barad-dûr was permanently destroyed as well.