A Flat Earth? Castle Stories Episode 14

Everyone knows that until “Columbus sailed the ocean blue” in 1492, every medieval sailor worth their salt thought that the Earth was flat and that sailing too far in any direction would lead to you plummeting over the edge. But like a great many things everyone knows, there may not be much truth behind the myth of medieval flat earthers. So what did medieval people believe about the Earth and its shape? And where did the idea of these medieval flat earth believers come from in the first place?