Idea of Great Wall Icon
The middle of the icon looks like an ancient castle with three pillars on top and a passageway underneath. Behind and in front of the central piece are parallel lines that run away and to the castle portion. Behind the central image the lines zig zag whereas the lines in front are smoother and linear.
This page shows the different variations of the Great Wall from an Icons8 icon pack. Feel free to resize and recolor the icon, and then download the icon in PNG format for free.