The temple. It was started in the 4th century B.C., the architect was Pytheos (who also designed the mausoleum in Halikarnasos-Bodrum).
It was to be the temple of Athena Polias. In the beginning Alexander the Great financed the building activities, but it was not finished until in the time of emperor Augustus (27 BC-14 AD). He was then honored along with Athena. The Ionic temple had no frieze but dentils above the architrave. The cult statue inside was based on the Athena by Phidias in Athens (at 2/3 of the size of the original). It had 6 x 11 columns and measured 37,15 x 19,50 meters. Vitruvius described it and through him it influenced design in the renaissance.
Some columns are still standing or have been reconstructed. There is plenty of material all around.