If you love history and adventure, you should visit Cao Cao's Underground Military Tunnel in the old city zone. This amazing tunnel was built by the legendary General Cao Cao during the Three Kingdoms Period (220-280), and it is known as the underground Great Wall. The tunnel is made of brick, clay and wood, and it stretches around the Dayushou mountain. You can explore the tunnel and learn more about Cao Cao's military strategies and life stories.