I'm having a 14 hour overnight stopover in Seoul in early December. I was looking to leave the airport and maybe see some of Seoul. Are there any reputable and good night tours of Seoul, especially walking tours?
You can take the Seoul City Night Tour. This is a non expensive city tour that departs at 8pm all days except mondays and city holidays. The problem is that you won't see all main city sightseeing, but it will give you an idea of the city.
If you are looking for something more interesting (and expensive) a friend was very happy with the night tour of I love Seoul. I've not tried, but it seems more complete. It starts at 5pm (I don't know if that's a problem for you) and must be 2 people to take part in the tour.