How to make a trench with landscape tools?


I want to make a trench like it was used or build in WW2 by german forces to fortify their position. Now I have a problem. I want to make the trench like you’ll see in picture 1 but it’s ending out like what you can see in picture 2. I don’t know how to get the terrain to a 90 degrees angle or similar to get a effect like you can see in picture 1.

I hope someone can help me with this issue. Thank you in advance!