Sardis was once the wealthiest city in the world, under the famous King Croesus (reigned 560 546 b.c. ), hence the expression rich as Croesus. 
King Croesus was in power when Sardis was quite wealthy.
mnli hypothesis: King Croesus was in power when Sardis was quite wealthy. premise: Sardis was once the wealthiest city in the world, under the famous King Croesus (reigned 560 546 b.c. ), hence the expression rich as Croesus. 