The history of Sardis in ancient times bears some resemblance to Jesus' description. It was once the supreme city of the region, the capital of the kingdom of Lydia, ruled by the famous king Croesus. His wealth was so great that even today people recognize the expression "rich as Croesus." But by the time John wrote Revelation Sardis had slipped to secondary status behind Ephesus, Pergamum, and even Laodicea. So in the first century the reputation of the city far exceeded its reality.
The parallel of history to text may be even closer. Ancient historians suggest that Croesus consulted the oracle at Delphi before heading into battle against Cyrus, the Persian ruler who would conquer Babylon eight years later. He asked the oracle if he should cross the Halys River to attack Cyrus or not. The oracle responded that if he crossed the Halys River, a mighty empire would fall.
Confident that he had a promise of victory, the king assembled an army and forded the Halys to meet Cyrus. The Persian leader overwhelmingly defeated him. But Croesus was not greatly concerned. Not only did he have the oracle's promise (he thought), but he knew that he could retreat to his impregnable fortress (Sardis) and raise an even larger army for the following year. But Cyrus pursued rapidly and surrounded Sardis before Croesus could gather the new army. The king was still unconcerned, thinking that Cyrus was vulnerable so far from his base and that the forces of Sardis would in time crush his army against the cliffs below the city.
The acropolis of Sardis sat on the top of Mount Tmolus. The sides of the mountain (which I have visited) are nearly sheer, almost like having walls hundreds of feet high. On such a height it must have seemed that a child could guard the city against an army. So Croesus retired one night in confidence that things would soon turn in his favor. He woke up to find the enemy in control of the acropolis and his kingdom now history.
What seems to have happened is that while the rock face below the city was almost perpendicular, a crevice had developed in the rock, allowing infiltrators to climb up one at a time to enter the city. Such an attack would work only if the defenders were unawares, so Cyrus' attack must have been at night. Along the lines of Jesus' message, destruction came upon Sardis like "a thief in the night."
Lord, heighten my spiritual alertness so I can be constantly aware of the angles of attack that Satan would use against me. May Your coming find me on guard.