Cao Cao saw that his troops were getting out of hand under this attack, so he called a council.

Liu Ye spoke up, saying, "Let us make catapults and so destroy them."

Cao Cao at once had models brought and set cunning workers to make these stone-throwing machines. They soon constructed some hundreds and placed them along the walls of the camp inside, just opposite the high ladders on the enemy's mounds.

Then Cao Cao's troops watched for Yuan Shao's archers to ascend the towers. As soon as the archers began to shoot, all the catapults began to heave stone balls into the skies and they wrought great havoc. There was no shelter from the falling stones, and enormous numbers of the archers were killed.Yuan Shao's troops called these machines "Rumblers," and after their appearance the archers dared not ascend the mounds to shoot.

Then Shen Pei, the strategist, thought out another plan. He set troops to tunnel under the walls into the midst of Cao Cao's camp and called this corps "The Sappers". Cao Cao's soldiers saw the enemy digging out pits behind the mounds and told the chief, who at once sought a counter plan from Liu Ye.

"As Yuan Shao can no longer attack openly, he is attacking secretly and is tunneling a road under ground into the midst of our camp," said Liu Ye.

"But how to meet it?"

"We can surround the camp with a deep moat which renders their tunnel useless."

So a deep moat was dug as quickly as possible, and when the enemy sappers arrived thereat, lo! their labor had been in vain and the sap was useless.