After reading another biography of Marlborough, I am annoyed by the fact the authors mention but do not describe the Lines of Ne Plus Ultra. They make it sound (I have read [Chandler][1] and [Konstam][2]) as if it was the ultimate system to defend France, and yet Marlborough 'defeated'/crossed it in a matter of weeks. 

Admittedly, he was good but... I don't know if it makes sense based on the ultimate level of defense it was meant to be. Even Wikipedia (at least in English) has nothing to say. So, **what do we actually know about the Lines of Ne Plus Ultra?** 

Note: I imagine I am held back by not searching in French to be fair, but I am not 100% convinced. 

  [1]: https://www.goodreads.com/book/show/93404.Marlborough_as_Military_Commander?from_search=true
  [2]: https://www.goodreads.com/book/show/9479840-marlborough?from_search=true