In France we visited the town of St. John Pied de Port. This town is right near the Spanish border in Basque country and has long been a trading post and fortified area due to its strategic location. I found more information about this fortress at this site which is run by an individual with interests in fortresses and siege warfare and I recommend that you review this link for more background and an overhead view of the fortified location.
I have not been to many fortresses but based on limited research this seems to be one of the best preserved locations. The citadel is intact (see below) and the city walls exist along with much of the earthworks.
Vauban himself. I am not a masonry expert but they must maintain these walls because they still seem intact and in good shape.
I highly recommend a visit to this city if you are interested in pre-Napoleonic warfare and if your significant other likes to shop, as well, there will be much for them to do since the town is very tourist-friendly.
Cross posted at Chicago Boyz