ISTANBUL — For 1,600 years, this city — Turkey’s largest — has been built and destroyed, erected and erased, as layer upon layer of life has thrived on its seven hills.

Today, Istanbul is a city of 13 million, spread far beyond those hills. And on a long-farmed peninsula jutting into Lake Kucukcekmece, 13 miles west of the city center, archaeologists have made an extraordinary find.

The find is Bathonea, a substantial harbor town dating from the second century B.C. Discovered in 2007 after a drought lowered the lake’s water table, it has been yielding a trove of relics from the fourth to the sixth centuries A.D., a period that parallels Istanbul’s founding and its rise as Constantinople, a seat of power for three successive empires — the Eastern Roman, Byzantine and Ottoman....