The Great Wall of China used to be a barrier against the Mongols. It then became a famous landmark for a few hundred years. Now, it's used to watch for large Deadman attacks coming from the outer areas of China or the inner. Undertaker Corps have set up men patrolling along the wall to each watchtower, and men on the ground either side of the wall stop any attacking ground forces.
It's possible for Sinners to sneak past, in or through, however it requires much skill or aerial transport.