The Changdeok Palace sits in central Seoul and is an intact, impressive former seat of the Korean monarchy. Unfortunately, the palace seems rather sterile because all of the artifacts and furnishings have been removed. It is a collection of mostly empty buildings. Unlike most World Heritage sites, you can’t just buy a ticket and stroll about the grounds. You have to be with a tour guide, and tours leave about every hour/half hour in different languages. Tours last about an hour.
This, along with the nearby Jongmyo Shrine are the easiest world heritage sites to visit in Seoul.