Second only to Pompeii, a visit to Ephesus is as good an introduction as one can get to ancient Roman civilization. Almost as astonishing as the site itself is that only 20% of the ancient city has been excavated so far. Allot at least a half-day for just an overview of the archaeological site and a full day for a comprehensive visit. In the heat of the summer, it's best to avoid the midday sun when the reflection off the stones becomes unbearable.  See the Overview for a complete discussion of the site.