[Formendacil]

Along the lines of Inziladun's first proposition, is it possible that we can distinguish between a literal Barad-dûr tower and a more general Barad-dûr ediface/city? I'm think of how Minas Anor/Tirith is both the seven-tiered city and yet is topped by the literal Tower of Ecthelion (who only ruled it as Steward something like 2500 years after Isildur and Anárion founded the city. (As far as that goes, though, I think we have to assume that there was a tower in Minas Anor from close to the beginning for it to have been named "tower" in the first place...)

By way of analogy, it's possible that Sauron founded the "city/castle/whatnot" of Barad-dûr ca. 1000 S.A. but that he didn't actually raise the eponymous tower until after he'd forged the One Ring. After all, since Mordor served as his base of operations for a number of centuries, it seems plausible that he would have had some permanent residence there. It's possible that the "Barad-dûr complex" was older than the tower which gave it it's name--and it's also possible that it wasn't until after Sauron "came out" as the Dark Lord that they realised he had a base in Mordor--and thus there would have been no reason to have a name for it in Elvish.

On the note of Elvish--and only tangentially related to the topic of Barad-dûr--does anyone have sufficient skills in Sindarian to distinguish between the meaning or nuances of "Minas" and "Barad?" I think of them as interchangeable, but I can't imagine Tolkien using two words for "tower" without having a distinction betwixt them.