Similar to the Routes of Santiago de Compostela, this is a site that spreads across the length and breadth of France. It consists of fortifications designed by the famous military architect Vauban, who was commissioned by Louis XIV. to devise a plan to defend France's borders and coastlines. The first site I visited was St. Martin de Ré, on the pretty Isle de Ré, just off La Rochelle. This was a bit of a disappointment, since the town itself is very nice, and there are also massive fortifications everywhere, but the citadel itself is still an active prison and can't be visited. The next visit was more successful - the triple fortification at the Gironde, built on both banks and an island to defend the port of Bordeaux. Blaye, on the northern shore, features a huge, quite well-preserved citadel, and you can take a ferry to the southern shore to the much less well-preserved Fort Médoc (about a 30-minute walk from the ferry terminal). The island on which Fort Paté sits cannot be visited, but can be seen from the ferry (even though you can barely make out any fortifications).

On a separate trip to Alsace, I went to Neuf-Brisach, just a few kilometres from the pretty town of Colmar (regular bus connections available). It was constructed in the completely flat Rhine plains, so Vauban was able to build his version of an ideal military city from scratch - a heavy fortification surrounding a small town with a garrison church and a parade square. If you are in the area, you may come across the Barrage Vauban in Strasbourg, a defensive bridge/lock on the edge of the Old Town which I think would make an interesting addition to the WH site.

A third trip brought me to Besancon in the Franche-Comté. The citadel dominates the city and includes several museums and a nice zoo. One ticket covers all attractions, and it is quite easy to spend several hours there. I would recommend to take a bus to go up and then walk down past the cathedral and the Porte Noire, a Roman triumphal arch. I also walked by the Fort Griffon by the train station (also on the list), but it houses a college and can only be seen from the outside.