Minas Tirith was the tower on the island of Tol Sirion that guarded the Pass of Sirion.
Minas Tirith was built around F.A. 60 by Finrod Felagund to prevent the forces of Morgoth from invading Beleriand from the plains of Ard-galen. After Finrod built Nargothrond he handed over control of the watchtower to his kinsman, Orodreth.
Minas Tirith was captured by Sauron shortly after the initial onslaught of the Dagor Bragollach, driving Orodreth out to flee to Nargothrond. Sauron then made it into a watch-tower for Morgoth from which he espied all that passed through the vale. As Tol Sirion became accursed, it was then called Tol-in-Gaurhoth, the "Isle of Werewolves".
Although the Noldor regained control of the Pass of Sirion until the Nirnaeth Arnoediad ("Battle of Unnumbered Tears"), they did not attempt to rebuild the tower.