Photo: Wikimedia CommonsThe red-sandstone Imperial Cathedral, or Kaiserdom, has towered over the old town for centuries and once crowned the Holy Roman Emperors. Its soaring Gothic tower can be climbed for a fine view over the rooftops and the river. Entry to the church is free. Climb the tower steps for the panorama if you have the legs, and look for the election chapel where the emperors were chosen, then explore the old town at its foot.