I've argued this point before with Martinez.
I understand where he's coming from but the fact remains that Tolkien, for all his descriptiveness, really does not give us all that many clues as to the precise apparel and architectural stylings of much of Middle Earth.
We really have not seen enough of Gondor to know just exactly how its culture will be depicted - a classical flavor or a medieval flavor? Or something not quite in either realm?
Osgiliath certainly had the smell of Roman or Byzantine ruins - but we'll have to see what ROTk has in store for the living Gondor.
Gondor seemed very much like the Byzantine Empire with Minas Tirith representing Constantinople, IMO.