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, Marlborough was skilled but... it doesn't make sense based on the 'ultimate' level of defense these lines are described to be in these biographies (without providing detail). English Wikipedia has nothing at all on this. So, what do we actually know about the Lines of Ne Plus Ultra—what were they and what was their intended function?

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