I visited the citadel and town of Mont-Louis, one of the inscribed Fortifications of Vauban, in the spring of 2013 as part of a day tour from Barcelona. Mont-Louis is located in the Pyrenees Mountains in southern France, and I thoroughly enjoyed wandering through the town and around the citadel. Vauban has been recognized as a superb military engineer, and it was fascinating to see the trenches separating walls in the fortified citadel, as well as turrets and towers providing line of sight and defense. As has been mentioned in another review, the main part of the citadel is closed, as it is currently a training ground for French Army commandoes. I was amused to see an obstacle course had been constructed along the rear rampart of the citadel in order to train the commandoes. Despite the closure of the central portion of the citadel, I was able to view quite a bit of the engineering Vauban employed in the construction of the citadel and the walled town, and it was well worth a visit. Also of note at Mont-Louis, but not recognized as part of the World Heritage inscription, is the world's first solar furnace, which is bright and visible upon arrival to the town.
Logistics: Mont-Louis can be visited via private transportation or tour, or via the French rail network's Little Yellow Train (le Petit Train Juane) which traverses the Pyrenees.