I remember this being a pain some years back when i played Shadow Wars on disc. So far I've found all but one of the pieces North of the wall (and all of the geodes for Twiddle). Still one missing and ive backtracked across the damned thing several times.

Is there a piece south of the wall? - Or does anybody know of a guide that gives actual positions? Everything I've found so far is pretty vague.

don't think they have positions. I havent played in a very long time but i think they can be south of the wall also. Also maybe if you've put buildings up, maybe you have a building over the spot. If thats a building location map, save then take down your buildings and search see if the spot is where a building was.