W/ all the talk of walls, here's one I'd never heard of: WWI, Dutch-Belgian border electrified fence
As the Netherlands had remained neutral, whilst Belgium was invaded (due to refusing the demand to allow the German army to pass through unimpeded, as an apparently necessary part of the Schlieffen Plan) and occupied, the occupying German forces wanted to control the border. They built a 2000-volt electrified fence, 125 miles long, 5-10 feet tall... and between two and three thousand people died by electrocution over the next 4 years trying to cross it -- well, they also summarily executed people who got within a few hundred yards of said fence, the wikipedia article does not break down death by electrocution vs execution.
https://en.wikipedia.org/wiki/Wire_of_Death
I'm not the world's most well-informed military historian, but I thought I had a pretty good layman's grasp on The Great War. But until today this one bit was unknown to me. There is always more to learn.
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