Groß St Martin is a lovely church from the 1200s. While it is much more basic than the cathedral,…read moreit is still quite nice and is well worth stopping by as you walk through the old town. The church itself is pretty, but the really impressive part is the excavation Roman ruins in the basement. They aren't huge, but it is a unique experience, and they are well signed and maintained. The church is free, but there is a ,50-cent fee to walk down into the excavation (they have laminated descriptions in four languages, but the signs are in German). Definitely worth the ,50 cents to see the ruins!