here is one of the vital spots of the Great Wall - the Pa La
Ling Gate leading to the famous Nankow Pass. Down this pass
swept many a horde of would be invaders of the realm, to be
checked and thrown back by the solid barrier they encountered.
Because the need of a garrison here was recognised long after
wall and towers to the far west had ceased to serve any practical
purpose and had fallen into dilapidation; this Nankow section
was frequently repaired, and the defensive wall was strengthened
by a special system of fortifications.
From Wonders of the Past