On a related note I just found Haldor's camp and was digging a moat around it to build a medium portal connected base.
I had thought the shield would stop anything but players from passing through but that appears wrong. A dwarf ran through it to get to me on the opposite side. When I walk into the shield area any mobs that had been coming for me act as if I never existed. Am I seeing this right?
The other thing more related to the op's issue concerns the moat I was digging. Before I saw the dwarf run through the shield I had intended to just dig a 'C' shaped moat with the open side up against the shield. As I neared the shield edge with the moat it seemed that I was able to dig inside the shield. I immediately stopped and came here to consult with the forum denizens. My concern is related to accidental destruction of the ground below the camp and messing up future trades. I know ground removal has some goofy mechanics in Valheim like floating chunks of ore, etc.