Medieval people already knew the Earth was round.
Educated people had known the Earth was a sphere since Aristotle in the 4th century BC; medieval textbooks such as Sacrobosco's 'De sphaera' around 1230 taught it as a matter of course. The claim that Columbus had to argue against flat-Earth believers in 1492 only became popular in the 19th century, partly through Washington Irving's novelistic Columbus biography of 1828.
Useful: Stories about 'ignorant ancestors' often reveal more about the storyteller than the ancestors.
Source: en.wikipedia.org