Government seems to go together with tourism in many countries. The UK has Big Ben at Parliament, and America has the Capitol Building. And of course, there are houses - the White House, Buckingham Palace, the Forbidden City, and Japan and Seoul's palaces.
The Korean National Assembly is the Capitol in Seoul, but with a short relatively nondescript history in a smaller country, this building is one that often gets passed over for more exciting stops around town... like the DMZ.
Access is via subway Line 9; get off at National Assembly Station and take exit 6. Facing the building, head right and follow the fence around the corner to the entrance to the Memorial Hall. It's not very well signed but it's basically the first available entrance.
The grounds themselves might be open to visitors; websites have mentioned the library, a nice pond in front of the library, a traditional style house in the garden, and the office building.
The visitor's center is open daily 9:00-5:00, and tours are given weekdays. Tours aren't given when Parliament is sitting or if there are security concerns. Admission is free. I would like to take a tour of the building itself sometime. On my visit, walk-up tours were possible for very small groups (mainly solo visitors like myself) but you should email (firstname.lastname@example.org) or call (+82-2-788-3656, 3664) at least three days in advance now to make a reservation.