The passes are the Great Wall’s doors. Located at key positions, their purpose was to screen the migratory stream. The walls surrounding the doors were twice as thick as the traditional ones. Many towers occupied this vulnerable place. The developed architecture allowed ten warriors to push back an army of a hundred men. The doors were set up some ten kilometres away from a big city. The double doors were made of enormous wooden beams, held by iron pins. They were closed by large rings. Above each door was a tree or a four level imposing tower. During the Mings’ time, the army had to watch over more than a thousand passes.   + photos ...