This was around the year 390 BC, when Rome was just beginning to consolidate into the empire it later became. The fledgeling Roman army was defeated at the Battle of the Allia, and the Celts pressed onward.
The people of Rome barricaded themselves in the highest buildings to wait for the end.
On reaching Rome three days after the battle at Allia, the Celts were surprised to find the gates of the city wide open. Suspicious, they wandered through the deserted town, always returning to the marketplace. The deserted town frightened them as no battle could.
In a courtyard, finally, they discovered several old men sitting motionless on ivory chairs. With their staffs in hand, they resembled Gods or guardian spirits. They were in reality the elder patricians of Rome, who had refused to flee to the Capitol with the other citizens to spare their share of the supplies.
One of the Celts got his courage up and plucked at the beard of one of the old men, who promptly hit the Celt on the head with his staff. This broke the spell and the Celts killed each of the old men. After that, the Celts felt free to plunder as they chose. They took spoils and set the town on fire.
Then they turned their attention on the Capitol Hill.
The Capitol Hill is a steep hill and hard to attack. Corpses began to pile up on the slopes. The Celts tried changing tactics, setting up a siege. But their supply lines weren't good and it didn't work. Disease began to ravage where hunger failed and the Celts tried sneaking into the Capitol.
This is where the legend of the Sacred Geese of Juno came about. The Celts, stealing over the wall by night, alarmed the geese who set up a mighty din. A former consul named Marcus Manlius hit the first Celt over the wall with his shield, and soon the Celts were heading rapidly down-hill.
After seven months, the Celts agreed to go away if the Romans paid them tribute of one thousand pounds of gold. The Romans agreed, and the scales were set up.
The Romans grew suspicious that the Celts' scales were faulty and accused them of cheating. At this point, the Celtic leader Brennus threw his sword on the weight and said, Vae victis, woe to the conquered.
In later times, the Romans would attain the upper hand first in Gaul then in Britain, but they would always be leery of the People Who Conquered Rome.