Something there is that doesn't love a wall,
That sends the frozen-ground-swell under it,
And spills the upper boulders in the sun;
And makes gaps even two can pass abreast.
--Robert Frost
Your Great Wall of Seoul is being undermined by tree roots, water and wind, @slowwalker, and yet it endures. Each generation mends it, because, "good fences make good neighbours", but ultimately a wall is there to keep some people in and others out.
Your wall in the modern era is mainly symbolic and part of the character of Seoul - the link to its glorious past.
The photos today are particularly striking, my friend, and the depict each step of your trek along the wall, unhampered by houses and dense population. You can truly see the wall as it is, along with its ancient foundations.
Outsiders can see a lovely fortress wall with window openings and parapets and observe the way it twists and turns and dips and rises following the hilly terrain. I too wondered at the task of erecting a great wall under such circumstances. It is quite an achievement, as was this post, @slowwalker.
Beautiful work!