The Temple of Artemis stands apart from the main Roman complex, below the acropolis and the mountainside. It was begun in the Hellenistic period and never fully completed, which somehow makes it more evocative: enormous columns, open sky, and the sense of a city whose ambitions kept changing.
Walk around it slowly. The surviving scale is enough to explain why Sardis belonged in the same conversation as the great Anatolian temple cities, even if much of the site now lies in fragments.