Changdeokgung Palace (창덕궁), built during the Joseon Dynasty and now a UNESCO World Heritage Site is a must-see in Seoul. It has various interesting buildings (halls, gates, etc.), but for me the most interesting part is the walk-through. The area it occupies is large, and the buildings numerous. As you can perhaps tell from the picture, it is much like walking through a maze: you enter one section, have a look around; notice a few different gates, take one and end up in another section; the new section is similar but different, and on and on. During the winter it is not crowded and you can enjoy a leisurely stroll, weaving your way in between the buildings without bumping into more than just a handful of people.