What is the reason for the Great Wall is "the world heritage site"?

The construction of the Great Wall can be traced back to the Warring States Period and has long history. It is hard to build such kind of defensive system on the steep mountains. The tough work made many people work on it. All the sections of the Great Walls are built by people not using many complex machines.
It is a miracle on the architectural history for not only China, but also the whole world.