The Basilica of Constantine in Trier was built in the 4th century and spent the Middle Ages as the residence of the Bishop of Trier. It became a Protestant church in the 19th century, and was burnt out in an air raid in 1944. A new organ was installed in 2014. Listen to it.
There is a long tradition in Germany of wooden carving, and an equally long folk practice of driving iron nails into trees for good luck. The First World War propaganda machine put these two together in the form of a massive wooden statue of Hindenburg in Berlin's Königsplatz, inviting patriotic Germans to drive iron nails into it. Source. See also.