Magnesia is the name of a prefecture in modern Greece and was also the name of an ancient Lydian city near Sipylus. The Turkish Magnesia is today known as Manisa and is in the region where Peter James has chosen as the location of Atlantis.
Plato also names his ideal city, Magnesia, in Laws, which was one of his last dialogues. For those that consider Atlantis to be just a concoction, I think that there is an obligation on them to explain why Plato would have found it necessary to invent two ideal cities. Furthermore, while no one has suggested that Magnesia was ever a real city, the acceptance of a factual historical basis to the Atlantis story has persisted since Plato’s time. On the other hand while Plato never claimed any historical basis to Magnesia, he repeatedly claimed that his Atlantis story was founded on fact.