I found this entry on a Middle-earth Wiki. It includes the history of Thranduil's halls.
reading that entry i still believe that the elves built it as they used it as a fortress, and because the doors could only be closed and secured by magic I think that strongly suggest that elves built that fortress.
personally i believe that elves built the halls of thranduil because elves can build many beautiful things, but I don't have any idea how long that would have taken
I would agree there was a strong defensive element in Thranduil's halls, since he was concerned about the Necromancer. Since the halls were modeled after Menegroth, the palace of King Thingol in the First Age, they were undoubtedly beautiful.