So. 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 and Konstam) 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.