Leave a margin
Metro last trains fall around midnight. The day's timetable rules. It is not an all-night service. Do not time Hongdae, Dongdaemun, Itaewon or Gangnam to the last one. Sleep on the same axis. Walk the night street to the close you already chose, with more than one train still in hand. At Dongdaemun, leave once you reach DDP. Do not transfer to a second night district to chase a train. The AREX express does not stop at Hongdae. On an airport day, do not count a last Hongdae train into the plan. If you miss it, stay in the district where you are sleeping. When one train is all you have left, do not transfer to another night district. Dongdaemun closes at DDP, Itaewon closes on the slope you picked, Hongdae closes at Yeonnam or the main street, and then you board.