Complex sentence: Sauron ruled Mordor from Barad-dûr; the greatest fortress ever built since the fall of Angband, and much of his personal power went into it.
Split into simpler sentences: Sauron ruled Mordor from Barad-dûr. The "Dark Tower" was the greatest fortress ever built since the Fall of Angband, and much of Sauron's personal power went into it.