Reverse Side of the World

The place where the evil dragon Fafnir finally arrived at the very end of the novels. The Phantasmal Races, having understood that the Age of the Gods was over, ceded the Earth's surface to humans and moved to this place. The world where humans currently live (including the laws of physics) is like a fabric that thinly extends across the surface of the planet. Beneath that fabric exist the planet known as “Earth.” On the other hand, the Reverse Side of the World is the world as it was before humans lived there… that is, the fabric of the era where Phantasmal Races strode the land. In other words, the Earth is the bottommost layer of the planet, and it is covered by the “Reverse Side of the World”—the place where the laws of the “world” as it once was reigns, and covering that is the “Present World.”
In the Reverse Side of the World, the Holy Grail cannot activate its function of “using the Third Magic.” In the first place, that function was for the sake of granting Shirou Kotomine's wish. In other words, it is programmed to target humans, and doesn't count Phantasmal Races as valid targets.
Jeanne d'Arc is a Heroic Spirit who, from the start, lost her human body long ago. Because of that, the chances of her reaching the Reverse Side weren't completely zero… However, that is merely the kind of “not zero” like, as a comparison, somehow managing to pass through a wall with the tunnel effect, but the Throne where Heroic Spirits Exist is cut off from the time axis, so she effectively had an infinite number of attempts to try.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia