This cache is located around Yongmun Station in Yangpyeong County. The Chinese characters for Yongmun mean Dragon Gate. Using the ITX or Mugunghwa trains, this station is only about 40 minutes from Seoul. It makes for a nice excursion outside of Seoul, especially if you're looking for a nice hike or just a quiet town.
The cache that you're looking for is a jar. Good luck and enjoy Yongmun!