Which part of Seoul's ancient city walls is closest to Hongdae by foot?

On a previous trip to Seoul I became very interested in the old city gates and city walls. A couple of the largest gates are famous and are close to the city centre. But the smaller gates and surviving sections of the wall don't seem to be well known at all.

I've seen the wall referred to by various names including "Seoul Fortress", "Fortress Wall of Seoul", "Hanyangdoseong", "Seoulseonggwak", and even "The Great Wall of Seoul".

Its Wikipedia article only mentions its history, no maps, no tourist info.

Most of the useful info I can find is in various blog posts: one from 2009, one from 2010. Maps seem to be impossible to find, especially ones which show the location of the walls along with locations of roads or the subway system. And even pasting placenames from the blogs into Google Maps usually doesn't help.

I know there is one part of the wall recently opened for hiking but it required you to bring a passport and is closed at 3pm.

What I'm looking for are the lesser known parts of the walls, no matter their condition. Many of the backpacker and guesthouse style accommodations are in the Hongdae area of Seoul and I like walking random streets. If any part of the wall is within about two hours walk from Hongdae that's what I'm looking for. Asking locals doesn't seem to work because they don't understand why I would walk rather than take the subway.