The Wall was a defensive structure used in some forgotten war, now worn away after most likely being overrun and forgotten. Along the ruins of the Wall you can find the occasional foundations of attached structures, and every now and then a small watch tower that has survived the ages.
This particular watchtower also has some siege constructions hidden beneath it, reached by a secret door in the floor of the tower (or another one that is covered by moss and lichen some distance from the tower). The siege constructions are old, dirty and damp, and one portion has collapsed from age and neglect, forming a sinkhole on the surface.
The Fort on the Wall was drawn in pencil on plain white paper, was lost for nearly four years, and then was scanned and contrast enhanced.