I don't much about Giessen really. My first month there I was engulfed in culture shock. After moving to Fulda, I drove down there every month or two to see my boss (that was often enough). I don't know anything about the silos there, but I know there were still flak towers in existence near various bases in Germany. If there was one thing the Germans did extremely well in WWII, it was build robust concrete structures. I also don't know about underground facilities in Giessen, but most places I visited (including Fulda), there were rumors of massive underground facilities. There was probably some truth to it, at least in some locations.