@davidjackson
I couldn't help laughing. That's the translation's fault. The word ( Talsperre ) does not only mean a dam for water, military it is a passage blockade.
The same as ( Westwall, Atlantikwall, or English Siegfried Line. Romans = Hadrian's Wall or Limes.
From wooden palisades mostly customs offices. From stone strategic blockades.
A nice example is the Mühlbacher Klaus. It looks like the VM drawing. But it was built around 1500. The older wall stood 200 m further down and was torn down at the same time. It is quite possible that the old wall is similar to the VM drawing.
But pure speculation, I did not find any pictures of the old wall.