The Ghost Fighter has sent his mirror image to the capital to spy on the humans' every move. The Skeleton Warriors have recieved the signal to climb the City wall.
Skeleton Warriors will climb up the walls, and you will need to kill them. There are also Defence Guards who will help you until they die. After killing 30 ordinary Skeleton Warriors, Elite Skeleton Warriors will spawn. After killing another 60 monsters, Promenade of Ghost Fighter will show himself.
Tip: Try to save as many Defence Guards as you can, and lead the Promenade of Ghost Fighter through the remaining ones. This will give you extra man-power, and sometimes he will focus them instead of your tank (gives your healer a breather).