Source of Lewis’s Narnia?

When Lewis wrote the Chronicles, he thought he had invented the name Narnia; later he realized it was a real place in Italy.

In Companion to Narnia Paul Ford lists ancient references to Narnia: “Pliny the Younger’s letter to his mother-in-law, in which he mentions the excellence of the accommodations of her villa at Narnia, especially its beautiful baths. Of all of these references, Lewis mentions only Pliny the Younger, in a letter to Arthur Greeves (They Stand Together, Macmillan, Collins, 1979, p. 171).”

After Lindskoog noted that there is no mention of Pliny the Younger in They Stand Together, Ford corrected himself. “Of all of these sources, Lewis mentions only the Annals of Tacitus, in a letter…. I will make this change in the fifth edition.”