A correction of the translation of Dr Zenker's answer: The ammonition plant in Siegburg produced only in WWI, not II!
Mary: In which POW camp do you think your father was? Hoffnungsthal or Siegburg? If you'll visit Siegburg you're welcome to visit both places with me!
About the cemetery: There is one which was used only for the friars, beneth the rose garden. It is not open for visitors and not noticable from the outside. I don't know whether this cemetery was used in WWII while the friars were not there. A second one. the Old cemetery, is also quiet central but has been full already before WWI. The third cemetery is at the town boundary, here are also parts used as military cemetary, fields for political victims of the Nazis, for russian and other slave laborers and also for victims of the bomb war. I don't remember any graves of POW, but it some years ago that I was there to make some photos. Nevertheless, this third one may be the most promising one.
Christoph
While looking for a translation of a word I just found out what the Memorial Day is., so these photos are my part to think of the victims of war.