Haigerloch Castle gained notoriety for an experiment conducted in 1944. Due to the allied forces advancing towards Berlin, the research group around Werner Heisenberg moved to far-off-the-grid Haigerloch to continue their research in nuclear physics. This experiment, called "B8", was set up in a beer cellar underneath the castle. B8 was a subcritical set up to measure neutron flow- it is not a "reactor" in truest sense of the word, since it was never designed to go critical, i.e. to sustain a nuclear chain reaction. In fact the experiment B8 never even got as far as the work done by Fermi in Chicago in 1942.In 1945 Haigerloch was a target for the famous ALSOS- missions. Nowadays the cellar contains a model of the B8 set up together with a small exhibition on the topic.
While the idea of Nazi super weapons, ( a la Captain America) being developed in a secret cavern makes exciting movie plots, I have to say this is all they have ever managed. And as a child of the 80es I have to say I am damn glad that the Nazis never got their hands on a working nuclear weapon