I can only say that Wolfgang and Joe are almost certainly right. In a military context, the word "Stollen" must refer to an underground tunnel.
The Christmas cake probably got its name because the marzipan filling is essentially wrapped in a fruit cake "tunnel". Ina usually makes some at Christmas - and very tasty it is too. I am surprised you don't see it in Münster, Wolfgang. You certainly see it everywhere in Köln!