Sauron builds Barad-durSauron begins building Barad-dûr (ca. 1000 S.A.) - In his bid to conquer Middle Earth, Sauron began building his great tower, Barad-dûr ("Dark Tower" in Sindarin) in the land of Mordor.  It would take 600 years to complete and became the strongest fortress since the days of Morgoth's Angband in Beleriand.

 

"c. 1000  Sauron, alarmed by the growing power of the Númenóreans, chooses Mordor as a land ot make into a stronghold.  He begins the building of Barad-dûr.

"c. 1600  He completes the Barad-dûr." ~The Return of the King (Appendix B)

"In this Age, as is elsewhere told, Sauron arose again in Middle-earth, and grew, and turned back to the evil in which he was nurtured by Morgoth, becoming mighty in his service.  Already in the days of Tar-Minastir, the eleventh King of Númenor, he had fortified the land of Mordor and had built there the Tower of Barad-dûr, and therefore he strove ever for the dominion of Middle-earth..." ~The Silmarillion (Akallabêth)