Changgyeonggung Palace was originally constructed in 1418 and has since then been rebuilt because of various reasons. The Palace grounds are a lot bigger than one would think by just looking at the gate from the streets of Seoul.
Cost for each adult: 1,000 Won (500 Won for children.) You can also buy a combination palace pass for 10,000 Won for all four palaces in Seoul (good for a month, valid for one visit to each palace.)
The Changgyeonggung Palace itself is beautiful but there is not much to look at besides empty buildings. The grounds, though, are even more spectacular. The huge pond in the back, which used to be two ponds until they were merged, was a picture perfect scene; I could have sat there all day.
Directions: Hyehwa Station, Seoul Subway Line 4, Exit 3.