Tower of Barad-dûrTower of Barad-dûr - Sauron's abode in Mordor.

 

"Darkness lay there under the Sun.  Fire glowed amid the smoke.  Mount Doom was burning, and a great reek rising.  Then at last his gaze was held: wall upon wall, battlement upon battlement, black, immeasurably strong, mountain of iron, gate of steel, tower of adamant, he saw it: Barad-dûr, Fortress of Sauron.  All hope left him.  And suddenly he felt the Eye.  There was an eye in the Dark Tower that did not sleep.  He knew that it had become aware of his gaze." ~The Fellowship of the Ring, Book II, chap. 10 (The Breaking of the Fellowship)