Barad-dûr was built by Sauron in 1600 of the second age. Barad-dûr's other name is Lugburz. Sauron ruled the whole Mordor from Lugburz. Sauron built Lugburz using his magical powers. And the vast majority of Lugburz used steel. People and Elves tried to destroy Lugburz many times but failed. But in 3441 of the third age People and Elves defeated Sauron. And so the vast majority of Lugburz was destroyed. But when Sauron became stronger again, he rebuilt Lugburz. And Sauron continued to live in Lugburz until the power ring was destroyed. After the power ring was destroyed, Lugburz was completely destroyed.