The myth of the brass ring, which is seamlessly forged into the iron grille, also has a legend. He is said to have gotten in like this:
The master Kuhn, who built the grate around the well, had a daughter named Margret, who was courted by his apprentice. But since he didn't want to give his child to a poor boy, he forbade this advertising and threw him out. It is said to have said something like: “Nothing will come of this! Nothing will come of it if you manage to make the rings on the well grate turn!” The master then went on a journey and the apprentice wanted to prove that he could do something and secretly made the ring. Then he cut it open, inserted it into the grid, soldered, hammered and filed until no seam could be seen anymore. Then he left town and never came back. When the master returned home, he realized that he had been too strict. He regretted the expulsion and would have liked to have had the skilful apprentice back and also given him his daughter, but it was too late and Margret cried her eyes out. One of the rings is considered a good luck charm; according to legend, anyone who turns it will be blessed with a child. The brass ring is usually thought to be a good luck charm, but many people in Nuremberg believe that the iron ring is the “real ring” and therefore the good luck charm.
According to another legend, the sculpture of the fountain was actually commissioned as the top of the tower of the Frauenkirche (also on the main market), but this was rejected due to a lack of lifting options. However, the Frauenkirche was consecrated in 1358. The legend was apparently also able to spread because Beheim had actually designed part of the vestibule in the Frauenkirche before the fountain was built and the graphic designs were also intended for the furnishings of the Frauenkirche
Source: Wikipedia