We Went to Namdaemun Market on a Sunday which probably wasn't a clever idea as the shopping centre next to it was closed. The market was in full flow though. I didn't personally find anything of interest to myself but there are lots of clothing sellers along with various other stalls and some excellent street food available for when you feel you can't walk any futher. Stay away from the stall selling the boiled/fried bugs as they STINK! I nearly wretched a few times just walking past haha!
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