Looking on a city centre map of Lille, I spotted a very interesting looking 5 point Citadelle at the edge of the old town, as handily shown by OSM:
I wandered over there, and saw some impressive looking walls and ramparts, but after a short walk round part of the surrounding park I couldn't spot anything about being able to go in.
Is the Citadelle generally open to tourists? And if not, are there any times when you can visit the inside of it?