I have been to the Fortifications of Vauban in Longwy during an 8 month evangelical missions trip to France in 1994. Longwy was our base town.
The Fortifications are awesome! I would walk around them a few times a week when the weather was nice. It was neat to see other people enjoying the area as well. I highly recommend checking them out if you're ever in that area.
I would also like to add that the French people were some of the kindest and friendliest people I have ever met.