One of the big draws for us to go to St. Goar other then the Rhine, are the castels. These aren't the Fairytale Castles of Hohenschwangau and Neuschwanstein, but the war fortress real deal. We found a picture of the castle as viewed from St. Goar in old times.
Rick Steves write up on the castle. I'm sure much more informative then what I'm writing here.
Oh before I forget. Once these mines were done extracting ore they were turned into defences. The roofs of the mine were covered in slate. Then the bottoms were filled with gunpowder. If a advancing army were in the field approaching the castle, the castle could detonate one or many of the tunnels. This would send explosives and slate shrapnel everywhere. According to the history books they did in fact use this, and it proved extremely effective.