hey TSC!
that's incredible! I've been to both places! to tell you the truth I grew up 30kms away from the hackenberg, and 1 of our favorite sunday afternoon occupation was to visit any and every fortifications around. we visited some that are open to public with set visits like the hackenberg and some that are close to the public... but don't tell anyone...

did you use the train in the tunel during the visit? and did you see the "surgery" rooms? frightening!!!
Have you been to douaumont?
I don't know for you but I was feeling very strange while in the fortification, the most threaling one was douaumont due to all the explanations, pictures and everything else you can see. the cemetery sight is also... hard to tell...
I grew up with that history very present around me, and I'm also interested by it, I had a neighbour who was at auschwitz, he couldn't sleep at night, we often found boumbs from the 2nd war in the fields almost everywhere in the region, etc...
next time you may visit the north of france there are many V1 launch site to visit and few V2 factory and launch site around Saint-Omer to visit, really interesting to see, and sadening regarding the numbers of people who died to build them...
and give me a shout next time you come to my country, we never know...