"The animal was gigantic, a stallion the size of a Percheron but with more graceful build, sleek and black as polished midnight. It was not tethered, though an elaborate fringed pair of reins hung from a headstall chased with silver and arabesques. On its back was a saddle, high in pommel and cantle..." (p. 14)
Having googled "Percheron," I now understand the significance of a stallion the size of one.
Holger dons the armor and rides the horse. He seeks hospitality from an old woman called Gerd who sounds like a fairy tale witch. They speak in a language that he does not recognize but nevertheless understands. She claims to live "...by the edge of the world." (p. 18) That might be literally true.
If this is a flat Earth, then how did it come into existence? Cosmos-forming processes would have to be entirely different. The flat earth of Narnia was directly created, sung into existence, by Aslan. There are some parallels between Three Hearts And Three Lions and The Lion, the Witch And The Wardrobe - apart from a leonine element in both titles, which I have only just noticed. Tolkien elaborately explains how the directly created flat Middle Earth became our round Earth. I suppose that a flat Earth has a place in our collective imagination. In Anderson's The Broken Sword, a ship leaves Midgard and enters Jotunheim by sailing North.