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.
--The Fellowship of the Ring, book II, chapter 10: The Breaking of the Fellowship
...at last, high in the upper cone, but still far from the reeking summit, to a dark entrance that gazed back east straight to the Window of the Eye in Sauron's shadow-mantled fortress.
--The Return of the King, book VI, chapter 3: Mount Doom