There are quite a few climbing gyms in Seoul, which is indicative of how popular the sport is here, and although choosing one to use regularly might just be a case of picking wherever is closest, there are actually quite a lot of differences between venues that are worth considering. Art Climbing Center, located near to Jongno 5 station on subway line 1, has just recently become a staple of my weekly workouts.

Located two floors below ground in a small basement, Art Climbing Center is certainly quite small. I'd guess the floor space in the actual climbing area is probably little more than seventy square metres, but the place feels light and spacious thanks to the use of bright colours and the fact that it is kept very clean. The cleanliness in this gym is actually one of my favourite things about the place. They use liquid chalk only, and it appears that the they also clean the holds, which means there is no dust in the air or on the mats.

To compensate for lack of space, the entirety of each wall panel has been covered with holds. This means that despite the size there are actually a multitude of set routes with an uncountable number of variations that you can create for yourself.
The main reason I use Art Climbing Center, over the wall which is actually closer to my home, is that it caters to all levels. I've been out of climbing for over four years and I was very frustrated with my first visit to Cho Kyu Bok's gym in Gwangjin-gu. I'm not levelling any criticisms at Cho's gym, which would have been my dream training ground four years ago, but at Art Climbing Center I have been able to set a recovery training regime that has been much more rewarding thanks to the wall layout - and the atmosphere generated by the presence of other beginners.
Whilst researching online before my first visit I had some concerns after reading that the gym was actually too easy, and that the favouring of large juggy holds exclusively over all others meant that there was limited scope for pushing yourself. That actually proved itself to be completely nonsensical. Even if you find the marked routes easy there are some insanely fun and tough bouldering problems that the regular gym users set one another.

The other gym users are also one of the big draws of this gym. Despite my extremely limited Korean, and their very limited English, they have been very welcoming. It seems to be the norm in Korea for climbers to take their training very seriously. I am basing all of my comparisons on experience that is around five years old, and limited to a few walls in the UK, but I don't remember people taking their physical conditioning quite as seriously within the wall. Every user at this gym, at least as far as it appears to me, runs circuit training style exercises between their efforts on the climbing walls. Despite all of this however, there is never a sense of pressure when you are climbing yourself. There is a total absence of show-boating and big egos that I always used to associate with bouldering walls.
The other users also spend a considerable amount of time in the wall. So although it might appear quite busy on weekday nights, you actually still get quite a lot of time on the walls because everyone goes at a slower pace. The more severe inclines are usually free - most of the waiting is just for the easier walls.

I have yet to see what the place is going to be like in Summer when it gets very hot outside, but it has been very comfortable in the Winter. It has been warm without getting too stuffy. I haven't noticed any dampness - and even when I have been the first person in on a morning I have not had to wait for it to warm up. Other gyms I've used have really suffered from the cold during the Winter months. This has all meant that the place doesn't suffer from the usual smells associated with indoor climbing gyms where everyone is sweating in their tiny little climbing shoes.
There are some good training aids in the wall as well - including weights, finger boards, racks and pull up bars. Male and Female changing rooms are separate - with more than enough lockers to store away your valuables (although I'd dare to say you don't have anything to worry about here).
Access is by elevator - don't take the stairs down past the Little Hanoi restaurant. If you try to access via the door at the bottom here you'll make a mildly embarrassing entrance like I did on my first visit.

Art Climbing Center is open from 10:00 to 22:00, Monday to Friday. Although it is closed on weekends they do run regular outdoor trips on Sundays. I'm not sure what beginners classes cost but the quality of instruction, from what I have seen, is very high (but only in Korean). A single visit to the wall costs 15,000 won (which is perhaps a little pricey if you're only going to hit the wall for a couple of hours), but a monthly pass is 90,000 - so you only need to make six visits to get your money's worth. This puts it level on cost with most other bouldering gyms in the city.
Of course, this gym is not going to do you any good if you're looking for roped routes - it is exclusively for bouldering - but it is a great place to train occasionally even if you are not a boulderer. The main things that I would say recommend it above other indoor walls are the cleanliness, accessibility for climbers of all abilities, and the welcoming atmosphere created by the regulars.
Although smaller than some of the other gyms it isn't quite as busy. Go on mornings if you really can't abide the crowds - even though I've never seen more than fifteen other people there at a time on weekday nights. Just remember that it isn't open on weekends, which can be quite limiting if you're looking to train two days on, one day off - or if you're flat out with work during the weekdays.