"Dongdaemun" is the old east gate of Seoul. The word is also synonymous with the massive market area that surrounds the gate, both above and below ground. Several of the nearby subway stations connect with underground shopping arcades. You can find almost anything down here. It's a true hive of commerce below street level.
If you're interested in the colorful, traditional Korean textiles, this is the place to come—blankets and hanbok (the billowy traditional dresses) make this a vibrant labyrinth. If you don't read Hangul, the Korean script, don't panic—most major directional signs are also labeled in English.