From my memory of Seoul (what's left of it after the soju), there's a lot of old historical landmarks. You can walk into a temple, and the entire city just seems to disappear. You're suddenly not in one of the largest and most modern cities in the world, but an ancient place with great weight of history behind it. Perhaps one of the historical sites could be useful?
On the other side, there's a huge chunk right in the center of the city carved out for the Yongsan post. I know how the American military can be, how well is that part of the city's underground patrolled?
