According to the Dragon Quest Timeline, Dragon Quest VI is in the Zenithian Trilogy while the first three Dragon Quest games are set in the Erdrick Trilogy. In Dragon Quest III it's revealed that Alefgard a part of Torland is in a dark dimension taken over by Zoma. Sound familiar to how in Dragon VI Mortamor's layer is in the Dread Realm. Fans have speculated that the Dread Realm becomes Nadiria while the the Cloudsgate Citadel became Zenithia. In Dragon Quest III, the Zenithian castle appears and there's an underworld. All three worlds must have existed before the timeline split in Dragon Quest XI. So, they must be alternative versions of each other. Alefgard is the underworld of the Erdrick Trilogy and in both timelines the underworld is taken over by a demonic villain. Alefgard which is a region of Torland must be an alternative version Nadiria also known as the Dread Realm. This would explain why the world map of Dragon Quest I and II is so small, it would explain why the geography is so similar across all the continents. Plus Nadiria has a large Plateau with one big mountain in the center, this is similar to the region of Rhone in Dragon Quest II. Torland being the Dread Realm in the separate timeline explains everything.