Seriously, where is Regiorra? It's a central point of reference for all the Radical Dreamers locations. Is it another island, apart from Zenan?
I have a theory based entirely on information in the Encyclopedia here. It says Viper Manor was unknowingly constructed on Zeal ruins. To me, that means look at where the Ocean Palace is on the map and find the closest relative position. It's the continent on which the city of Medina resides, near the south, where Melchior lives. Regionna could be the name of the continent. After all, it's widely accepted that the Zenan continent is called Zenan and not Truce, the city. Maybe he just owns a little shack and that house you see on the CT world map is Viper Manor? Maybe that's a stretch. But think about it.
We do know of at least one place where Zealan ruins are canonically known to exist: the Forest Ruins north of Medina. Those ruins also housed an underground vault in the prerelease version of Chrono Trigger. Once the magical pyramid was removed by Crono, the site would simply appear to be a large stone slab; perfect for the beginnings of a foundation. Although the vault was removed from the final version of the game, it's reasonable to think that it might still exist and was discovered when construction of the manor began. It may only be circumstantial, but it's also worth noting that Lynx in Radical Dreamers employs large numbers of goblins as guards; these could easily be recruited from Medina. Assuming the scene in Radical Dreamers showing Viper Manor (and the nearby town) is from the vantage point of the heroes on a cliff on Heckran Cave mountain, the geography would roughly fit, especially considering artistic liberties were probably taken.
Here's a picture to demonstrate what I meant.
As I said, the geography is a rough fit, but the only direct contradiction I see is that the blue line in the Radical Dreamers picture should be looking down on land. Perhaps that didn't look dynamic enough and it was changed to water to make the scene more impressive. Unfounded Speculation: The crenelated structure on the horizon is also a mystery. It could be a large dock on Medina's southern shore; possibly for the ferry Bandeau was going to establish. He mentioned that the ferry was going to run between Medina and Truce, which would seem to make Medina's western shore a more likely location to build a dock, but perhaps the plans were changed due to the fall of Guardia and the rise of Porre.
From: Theory (Locations)