Temple of Zeus Olbio, (Zeus Olia Tapınağı) built under Seleucus I Nicator. Only the peristyle remains: about 30 columns of 12 meters surrounding the central part. In Christian times the temple was transferred into a church, some signs of this are still visible. The holy area was larger than the temple itself, but also contained two crossing boulevards, from the first century AD, that are still the main thoroughfares.
The University of Freiburg (Germany) under professor Detlev Vanagat (or something like that, I got this from the very friendly guide. I was the only visitor that day, one reason why I always travel off season) is inventorying what remains and may well start reconstruction (plenty of material seems to be lying about).