Complex sentence: Although Barad-dûr was captured and levelled after Sauron's defeat, the foundations could not be destroyed while the Ring still existed; because of this the fortress was more quickly rebuilt when Sauron returned to Mordor thousands of years later.
Split into simpler sentences: Although Barad-dûr was captured and levelled after Sauron's defeat the foundations could not be destroyed while the Ring still existed. Because the foundations were intact this meant the fortress could be more quickly rebuilt when Sauron returned to Mordor thousands of years later.