Soldier Island is a fictional island that just so happens to be based off of a real place, Burgh Island on the South Devon coast. Not only does Burgh Island contain a suitably large mansion, but the whole place is cut off from the mainland at high tide, making for a rather foreboding setting. Perfect for someone looking for a mystery novel inspiration.
One of the first descriptions of the island that we get is when Vera looks out across the ocean:
She had pictured it differently, close to shore, crowned with a beautiful white house. But there was no house visible, only the boldly silhouetted rock with its faint resemblance to a giant head. There was something sinister about it. She shivered faintly. (2.80)
The set-up suggests that Soldier Island is not quite the friendliest place in the world. Yeah, you can say that again. Moreover, they’ll be cut off from the mainland and left to the whims of their hosts. The isolation is figurative as well as literal: the island setting means that the characters are cut off from their everyday lives and transported to a habitat with its own rules and peculiarities:
There was something magical about an island—the mere word suggested fantasy. You lost touch with the world—an island was a world of its own. A world, perhaps, from which you might never return. (2.193)
No police officers, no neighbors, and no cars driving by to save them if something goes wrong. Just a big house and a madman. Fun!
Nowhere to Hide
The other peculiarity about the setting is that it’s totally open, with no caves or crevices where someone can hide either on the island or in the house itself. Instead of describing a creepy old Victorian house, Christie opts for a modern (well, modern to those times) house without much in the way of hidey-holes and cupboards under the stairs:
If this had been an old house, with creaking wood, and dark shadows, and heavily paneled walls, there might have been an eerie feeling. But this house was the essence of modernity. There were no dark corners—no possible sliding panels—it was flooded with electric light—everything was new and bright and shining. There was nothing hidden in this house, nothing concealed. (5.68)
The whole setting makes it so that the characters have nowhere to hide, are cut off from society, and have no illusions about the fact that the killer is one of them—and that their actions have brought them here.