The tower and dungeons of Sauron in Mordor. Baradur was rebuilt
with the power of the One Ring after the year 1000 of the Second
Age. When Sauron was vanquished by the Last Alliance, Barad-Dûr
was destroyed, though the foundations remained as the One Ring
was intact. It was rebuilt by Sauron and finally destroyed in the
year 3019 with the destruction of the One Ring.
It was said by Gandalf that "those who pass the gates of
Barad-Dûr do not return". (LOTR.313)