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Well I'm pretty sure I found it by pure coincidence about a week ago on my way bag from a many-hours-long random walk around Seoul.
From Hongdae it turns out you can walk pretty much directly East following the sun if it's morning or away from the sun if it's afternoon. If you do that following the bigger roads you'll tend slightly North but you'll end up coming toward the main entrance of Gyeongbokgung palace from the side.
On the way back for variety I came a different way, also randomly and in the dark. This is where I found myself:
This part of the wall is obviously a pretty recent reconstruction and might not even be very accurate but it's obviously in pretty much the original place. There's a path going steeply up the hill and the wall did connect the tops of the four mountains surrounding Seoul's old centre.
The only problem is the signs are all in Korean and translating the labels is not giving me much helpful other than "current location", "Culture and Sports Center", and the name of a church, none of them turned up anything definitive searching Google Maps in English or Korean. The location is the tip of the bottom left point of the dark area on the map but I'm almost certain that North is not at the top of the map - it's rotated for best fit. Here's a close-up of the map which shows this edge of the wall along with the modern roads etc:
There was also this schematic map of just the wall without any details of the modern city, also only in Korean:
I'm uncertain of the position on this map though the bottom end of the red line seems sensible.
If anybody can pinpoint this location from the maps and the Korean labels it would be greatly appreciated.
As it was late and cold and I didn't expect to be here I didn't follow the wall path up the hill because it was the wrong direction for getting home. But as I headed off I finally found one sign in English: