Just took a look at the bunker-nrw.de site...very interesting!
Of course, there are many of these old bunkers and industrial ruins throughout the Ruhr and NRW area, some known, some forgotten.
During the early 1970's I lived in several small towns and villages out along the Hoennetal, not far from Hemer and Iserlohn, and out towards Balve.
The Hoennetal is a limestone "V" valley, very ancient, and is of course riddled with caverns. It also had it's share of now abandoned bunkers, if memory serves me correctly. To us (Canadian families) these bunkers were just part of the landscape and history of the area and we really didn't give them much thought.
There does seem to be a growing interest in exploring these places in recent years. Here in Canada, there is a television program that runs on the History Channel -- Underground Cities I think it is called -- which explores the "secrect cities" built under many modern European cities, some dating back to Roman times, others circa WWII, etc. Very interesting.
I know Iserlohn was (and is) a major industrial centre and for sure there are many bunkers there and in Lethmathe as well.
Same for Dortmund, although I am not as familiar with the city there, despite having visited it many times and gone to the Cornwall School (British) there (now the Europa Schule).
Thank you for posting this site, Bjoern!