I visited the Gyeongbokgung Palace during winter, well, now that it's summer we decided to hike around the palace. Previously, Mt. Bugaksan, the mountain behind the palace was not open to public due to military vulnerability (someone could attack from there), but now, tourist and locals can go. The heavily guarded trail was not for the unfit, furthermore, the weather was hot and humid. At some view-points along the trail it was not allowed to take pictures of the view, if a guard sees you taking a pic, he will follow you and ask you to delete it in front of him, I had to delete some.
The Seoul City Wall links several mountains and has a total length of 18.2Km. Most of this wall was built in 49 days in January and February 1396. 118,000 people from all over the country were brought. This wall is intended to defend capital that King Taejo built. 872 people died building the Wall.