So cheap hence the crowd is humongous T_T. Afri and I passed the skating rink after visiting Doksu Palace (Deoksugung) last week. At the time we were there, the rink was closed for break around 30 minutes after every 1 hour skating session. Seoul Plaza Skating Rink is just right across Doksu palace (Deoksugung) and you may go there by subway for City Hall Station, line 1 (dark blue), Exit 5 or 6.
Official website: Seoul Plaza Skating Rink
Check out this Naver blogger Hyunokmun for more details (in Korean).