I have been to this WHS in 2008. Koguryo kindom was not an empire since its kings were always authorised by Chinese emperors. The most visitable place is Ji'an, Jilin province. Besides of some ancient ruins, you can see a big stele "Haotaiwangbei", which is famous for its Chinese calligraphy. Wukuifen is an ancient tomb with many mural showing how ancient Chinese understand the universe, the Chinese legendary emperor Fuxi and Shennong was pictured on the wall.