Inside, you'll find some 3 floors of nice rooms and baths, moderately quiet areas to sleep, and separate levels for the sauna and jjimjil. For 7,000 won or less (depending on what time you arrive) you can even put your things in the lockers, get ready to shake your goove thing, and ask them to keep your key at the front till you arrive again in the wee hours. For a jjimjilbang, that's pretty rad.
서울시 마포구 서교동 371-10 번지 B1층
Seoul, Mapo Gu, Seo-gyo Dong 371-10, B1
Open 24 hours a day/ 7 days a week
This sauna is exceedingly easy to find. Despite this, my bf and I wandered around for over an hour trying to remember where it was! Now we know, and we will never stay anywhere else in Hongdae. If you leave the Hongik University subway stop (line 2) from exit 5, just walk straight along that street until you get to the large, tall Gyu Su Dang wedding hall (규수당 웨딩홀) building. The jjimjilbang is inside that building, down one level. Easy.