This article has an unusual and persuasive idea as to what all those spirits are. It really starts about halfway down.
I had always assumed the Watchers were something of Sauron's work. I hadn't really considered how the tower was originally Gondor's work, although that seems to make it pretty clear.
Chancellen, do you have any more examples about the Numenorean's "alien" architecture? You only mentioned Orthanc. The instance that always impressed me the most was Minas Tirith, and that seems a far cry from the watchtower of the pass.