The Great Wall of China was made to protect the people in central areas from being invaded by the northern nomadic tribes.

But there are no records of any great battles that involved the Great Wall.
If this were really a defensive wall against threats from the outside, then there would only have been battlements on one side of it. But the crenellations are built on both sides, so whoever was standing on the wall was defending it from attackers on either side. This structure must be a road, not a boundary wall.

Hello George R, if you look carefully, you will find one side of the wall is lower which was built to protect soldiers on it from falling down. Also, along some sections like Simatai, there is wall only on one side.