Seodaemun/Mapo is in northwestern Seoul, by the banks of the Han River.
It is most notable for having a high concentration of colleges and universities, resulting in a distinct, youthful culture, especially in the twin districts of Hongdae (홍대) and Sinchon (신촌), separated by a single subway stop, a ten minute bus ride, or a twenty minute walk. These two districts are home to some of Seoul's most boisterous nightlife, and Hongdae in particular is unique for having a strong underground indie music and arts scene, a welcome break from the sometimes unbearable monotony and conformity of the rest of the city. This area also has Seoul's 2002 World Cup Stadium.
- Seodaemun Prison, 101 Hyeonjeo-dong, Seodaemun-gu, (Metro Line 3, Dongnimmun stn, exit 5). Tue-Sun 9:30 AM-6 PM (5 PM in Nov-Feb). Originally built in 1908, the prison became infamous during the Japanese occupation, when it was used to torture, starve and execute Korean political prisoners. Actual prison cells, wax figures and videos are used to demonstrate the shocking brutality; most signs are only in Korean, but volunteer guides can describe the sights in English. Obviously due to the content, this site is not suitable for young children or those of a sensitive nature. ₩1500.
- Ewha Women's University. At the front gate of Ewha Women's University, visitors can find a dense market geared towards young women. You can find stores that sell clothes, shoes, hats, handbags, and so on. There are also clothes for men. Recently franchise stores have started to move into the area.
Hongdae is full of restaurants and street food, most opening up in the early afternoon and many staying open until the next morning.
The nightlife is why you come here! This is where travelers will find some of the best nightlife anywhere, and will discover the reason why Koreans are some of the heaviest drinkers in the world. Even on weekdays, Sinchon and Hongdae are loud and alive well into the early morning.
Sinchon (신촌), home to universities including Yonsei University (연세대학교) and Ewha Womans University, is a great place to soak up a more Korean environment. Not to be confused with Sincheon, which is in Songpa at the other end of the city, Sinchon is set up like many Korean entertainment areas in which bars, clubs, restaurants, karaoke boxes and sometimes even motels are structured in a grid-like fashion. The only way to familiarize yourself with the area is to stroll the alleys and discover all of the different places. Korean bars tend to be rather anti-social compared with their Western counterparts, with people sitting at tables with friends and not tending to mix. Yet a number of Western-style bars in the area have opened up.
- Woodstock. Around since 1991, the bar has prompted a slew of copycats but is the best place to hear classic rock and pop. The sound system is awesome and the owner/DJ knows his stuff. Expect large crowds Friday and Saturday nights and seeing people dance next to their tables. Great place to mix with Koreans of all ages.
- Nori Ha Nun Saram Dul. A basement bar infamous for both its great rock music and its decrepit interior with writing on the walls. Difficult to find and almost impossible to get a seat after 9 p.m. Secure valuables as theft is not uncommon in the bar.
- Beer O'Clock.  This Canadian-run bar serves great pizzas and other grub in addition to showing sports on the big screen and offering darts. Weekends are pretty wild at this joint.
- Neighborhood A New Zealand owned craft beer and pizza pub serving quality craft beers and pizzas daily.
Hongdae (홍대), short for Hongik University, is the premier club area in Seoul by far. Located around Hongik University, clubs and bars are strewn everywhere around the place. The clubs aren't near the station but aren't hard to find. The most popular clubs are:
- 18th Street Saloon Western-style pub owned by two Americans and a Brit -- the only one of its kind in Hongdae. Always good tunes, reasonably-priced beer and very international. Sports, CNN on the TV. Imported draft beers (Guinness, IPAs... even hard cider!) 4,000-6,000. 
- Cocoon Hip-hop and dance music. Entrance: Thursday ₩10,000, Friday/Saturday ₩15,000-20,000.
- nb (noise basement) Hiphop.
Some other popular clubs include
- Club Saab Small Hip hop Club
- Club Tool House Music club
- Club FF Rock music club that is popular with foreigners
- S Club hip hop club that gives free entry to all foreigners.
- Tin Pan A club/bar with cheap drinks and a dance floor. Only Koreans are allowed.
On a Friday or Saturday expect all of these to be packed tight. Last Friday of every month is Club Day where ₩15,000 will get you into those clubs who are part of the "Club Day". Expect a packed crowd. A number of bars are popular with foreigners here too:
- Bricxx A hookah lounge with an intimate atmosphere and a large mixed drink and wine list.
The place is huge and you could party for a whole week in all the bars and clubs. The best way to see it is to stroll around and find something you like. An interesting note: the entire club district of Hongdae is officially off-limits to U.S. military personnel, Federal employees and their dependents. However, it is not unusual to see crew-cuts here on the weekends since it is rarely enforced.
In warmer months, don't pass up the closet-sized B-Dan on Hongdae's main strip, which offers up take-out draft beer by the plastic cup.
More recently, a number of LP bars playing classic rock and pop has opened up. One to look for is Suzie Q, which is near the 7-11 around the clubs.
Because of this area's youthful nature, there are a large number of hostels and motels in this neighborhood, particularly south of Sinchon station. Here's a few of them:
- Seoul I Guesthouse - Hongik University/Hongdae, 561-61- Yeonnam-dong, Mapo-gu (7 minutes from Line 2 Hongik University stn exit 2), ☎+82 70-8779-6161, www.seouliguesthouse.com email; firstname.lastname@example.org. Easily accessible from the Subway/Metro Line 2 green line and the Airport Express Train. Located in Hongdae (clubs,bars) and close to other famous tourist attractions (Myeongdong,Insadong, Gyeongbok Palace,Han River) Awesome friendly multi-language speaking staff. Staff can help book DMZ tours, provide info on K-pop Concerts and other tourist attractions. No Curfew, Free breakfast (Coffee, Tea, Toast, Butter, Eggs, Jam), Free guest computers, free hi-speed wifi, Free use of Bicycyles (3 available), Free Security Lockers, Air Con + Heating, Large common room with Big Screen TV w/ Cable + Korean Movies, Kitchen w/ stove,fridge,oven,microwave available for use, Monthly BBQ parties. Mixed Dorm, Female Dorm available from 17,000W/night. Private Rooms Available as well. Discounts for long term stays
- Backpackers Korea - Sinchon, 106-105 Nogosan-dong, Mapo-gu (4 min from Line 2 Sinchon stn exit 7), ☎+82 2 715-6277, . Friendly cool English, Japanese, Chinese speaking staffs, Free breakfast (Toast, Jam, Peanut butter, Eggs, coffee and tea), free wifi, free international phone call, free laundry, A/C, hot water, rooftop terrace, common room with kitchen. All rooms are equipped with desktop computer, refrigerator, big flat TV with cableTV on the wall. private from ₩55,000/night with own bathroom.
- Bangrang hostel, 397-12 Chungnim-Dong, Chung ku (Metro Line 2, Chungjeongro stn, exit 5), ☎ +82 2 6614-2246, . checkin: 2PM; checkout: 11:30AM. A brand new hotel,opened Jan 2010. distinguished interior. 1 min from subway station, 2 min from airport bus stop. ₩18,000~. edit
- Blu Guest House, 464-63 Seogyo-dong, Mapo-gu, (Metro Line 2, Hongdae stn, exit#1; Line 7, Hapjeong stn),☎+82 70 7692-9484, +82 11 9921-5621, . Free wifi, free laundry, aircon/heating, lockers, breakfast, no curfew. From ₩18,000.
- Chocolate Tree, 109-91 Nogosan-dong, Mapo-gu (4 min from Line 2 Sinchon stn exit 7), ☎+82 2 3275-1115, . Friendly and adorable English, Chinese speaking staffs, Free breakfast (Toast, Jam, Peanut butter, Eggs, coffee and tea), free wifi, free international phone call, free laundry, dryer, A/C, hot water, rooftop terrace, common area with kitchen. All rooms are equipped with desktop computer, refrigerator, big flat TV with cableTV on the wall. you can get some chocolates at the reception at anytime. private from ₩50,000/night with own bathroom.
- Hongdae Guesthouse (홍대게스트하우스), 159-6 Dongkyo-dong 159-6, Paradisetel unit 302, Mapo-gu (Next to Line 2 Hongdae stn exit 1), ☎ +82 2 336-0003 (email@example.com), . checkin: anytime; checkout: 11AM. Clean, spacious rooms with high ceilings, ondol heating, aircon, hot water. English spoken. From ₩19,000. (one night,2 months) edit
- Kims' Guest House, 443-16 Hapjeongdong, Mapo-gu, (Metro Line 2/7, Hapjeong stn), ☎+82 2 337-9894, . Comfortable guest house run by a friendly English-speaking family. In the western part of Seoul, 10 min walk from Hapjeong subway station. Share kitchen, toilets and showers and have free use of cableTV, washing machine and internet. Breakfast (jam, toast & coffee) is included. No curfew. Discounts for stays over 1 night. Dormitory ₩20000, single, double and triple rooms; ₩27000/₩37000/₩47000, all have A/C and heating.
- Lee & No Guesthouse, 561-29 Yunnam-dong, Mapo-gu (Sinchon), ☎ +82 2 336-4878, . checkin: 1PM~9PM; checkout: 10:30AM. Near Hongdae, has four-bed dorm rooms and doubles. ₩22,000. edit
- Miso House, Unit 201, 176-14 Dongkyo-dong, Mapo-gu (Exit #5 at Hongik University station, next door to the cafe on your left as leaving the exit.), ☎ +82-10-7929-8327, . checkin: flexible; checkout: 12pm. Small and friendly owner run guest house within minutes walk of Hongik metro and Hongdae restaurants, bars and attractions. Free breakfast, wifi, computer, TV and laundry. Dorms and private single/double/twin available. ₩20,000. edit
- Soul Guesthouse, Unit#505, 159-6 Paradisetel,Dongkyodong, Mapogu,Seoul, (Metro Line 2, Hongdae stn, exit 1), ☎+82 10 5137-0604, . Female Dormitory. ₩20,000 per night per person. Clean and close to subway. Located at central Seoul.
- Seoul Walker Guesthouse, 441-31, Hapjeong--dong, Mapo-gu (nearest to Hapjeon Station, line 2. Directions are on the website and business cards.), ☎ 070-4028-6904, . checkin: 2-10 pm; checkout: 11 am. The owner, Mr. Yoo, speaks Korean, Japanese, and English. He is very friendly and always asks where you will go (in the morning) and where you went (at night). His young daughters hang around the guesthouse while he cleans. Laundry service is available for 5,000/cycle, which he will operate. He offers taxi service to and from Gimpo and Incheon airports, for 30,000 and 60,000 W, respectively. He has many pamphlets in different languages, books, a computer and printer, and WIFI. KakaoTalk and KakaoStory are available for communication. No curfew, but keep silence after 11. There is a code lock for security. 14,000 (dorm)-70,000 (family). edit