There's a saying that's all but been eliminated from our vernacular that calls a person "as rich as Croesus".  King Croesus of Sardis was, at one time, the richest man in the world.  This, owing to the Pactolus River- then the main source of naturally occurring gold in the known world- that flowed through the city.

Sardis was largely considered impregnable.  It contained a sizable citadel built into the mountain the city nestled against, high walls, and was situated in a manner that its only gate could not be accessed without those approaching being spotted long ahead of their arrival.  So it was with these known obstacles that Cyrus the Great, of Persia decided to attempt to capture the city.  After all, the city's treasuries held more than perhaps entire empires had known.  But Sardis was a tough nut to crack, and her people knew it.

"I know all the things you do, and that you have a reputation for being alive—but you are dead."

Cyrus brought the Persian army to the foothills of the mountain and set up camp.  He then called all his men together and promised one of them a very hefty reward for the man who identified a way for them to breach the walls and enter the city.  It took many days, but eventually a footsoldier informed his commander that he knew how to get in.  The word spread all the way up to Cyrus and the man was given an opportunity to present his plan.

The soldier reported that the gates were always guarded by two men every day and night.  He also said he had watched as one of the guards had accidentally dropped his helmet from the walls.  To reclaim his lost helmet, the guard walked outside the city and down a small footpath that was not visible from the angle that the Persians had on the city.  The man's plan was simple.  They simply had to wait until both of the guards fell asleep, and they could send a battalion up the footpath to gain entrance to the city.

"Wake up! Strengthen what little remains, for even what is left is almost dead."

Cyrus was quite pleased with the man's plan, and so they set to waiting and watching.  Eventually, a few days later, sure enough both of the guards nodded off.  With a wave of his hand, the Persians dispatched men to scurry up the footpath.  With no resistance at the gate, the men had little trouble gaining access.  And thus, the Persians ended up sacking the city and walking off with all of the Sardis treasury.