The Great Wall is not just a wall. The Great Wall is made up of three components: passes, watchtowers and walls. Along the wall are watchtowers, where soldiers used to keep watch. Fires were lit on the top of the towers as a warning when the enemy came. 

Other defensive works such as forts, passes and beacon towers were built along the Wall to house auxiliary soldiers, store grain and weapons, and transmit military information. 

Great Wall Hiking, trekking and camping
Great Wall Hiking
Great Wall Camping
Great Wall Trekking
Hikes around Beijing