Barad Dur, the Dark Tower, was built by Sauron. Its construction started in !000 Second Age and took 600 years to be completed. It was the greatest fortress to ever exist in Middle Earth, apart from Angband. It was razed after the War of the Last Alliance, and rebuilt after Sauron came back to Mordor. It was here that Gollum was tortured. It also has pits and caverns underneath it, that probably serve as prisons or dwellings. IT was destroyed with the destruction of the One Ring, like Sauron.