Excerpt from wikipedia.org/wiki/Bergfest_Dilsberg:
The castle tunnel was created using the separating joints in the rock, which is why the base and ridge are tapered. The tunnel was driven into the mountain from the outside and was supposed to hit the well shaft after 65 metres. However, the miners went astray several times. Therefore, the total length of the tunnel is 78 meters.
Dilsberg local researchers previously assumed that the tunnel was built so that the castle crew could escape to the open during sieges. Today, however, it is assumed that the corridor was a ventilation tunnel.
After the tunnel had served its purpose of protecting workers from poisonous gases, it was temporarily filled in. The legend of the underground passage that is said to lead to a Neckarsteinach castle under the Neckar was then formed. The writer Mark Twain mentioned this legend in his travel story A Tramp Abroad (in German: Stroll through Europe). The story inspired the German-American Fritz von Briesen to travel from New York around 1900 to find the tunnel. He let himself be abseiled and actually found the tunnel exit. With his financial help, the well and the tunnel were cleared of rubble in 1926 and the beginning of the tunnel was exposed in the forest.