A little below Palm's Castle in Mühlhausen there are a few walls and three gates in the sloping terrain. These form the entrance to the 'pioneer tunnel. The total of three tunnels of the 'Pionierstollen Mühlhausen' were driven into the mountain during the war. After the heavy bombing raids in April 1943, construction of the first tunnel began in August of the same year. This was named Pi 22. This is the western gallery. In October 1943, construction of another tunnel began, the Pi 87, today's middle tunnel. After completion, the post office and the pension office were housed in this. It is also known as the fat cat gallery. The tunnels are connected to each other. So-called caverns branch off from the main tunnel, which at that time were demarcated with slatted crates.