India and the Yorkshire Moors
One of the most striking things about The Secret Garden is that there are so many large, empty houses. Think of Mary sitting alone in the first chapter in her parents' empty bungalow, not realizing that her parents have both died and all of the servants have either died or run away. Or when Mary first explores Misselthwaite Manor's countless rooms and finds a portrait gallery and a family of seven mice but not much else.
Even when Colin joins Mary to explore the house one rainy day in Chapter 25, it feels endlessly empty. The two of them have lots of fun looking at the portraits and playing with the strange ornaments they find, so there isn't the same impression of loneliness that haunts earlier descriptions of these empty houses. But even when Colin and Mary are happy in their huge homes, there is still this weird sense that they are more or less alone against the world.
Obviously, it's a fun setting for a novel to put a bunch of kids into a more-or-less empty castle so that they can explore. But we also think that these huge empty rooms are supposed to remind us of a sense of scale: Colin and Mary are both small and vulnerable compared to the larger world they're trying to learn to navigate on their own. They have a much easier time figuring out what to do and where to go when they have each other, but they are still kids left more or less to their own devices.
So if Mary hadn't found the Secret Garden, and if Colin hadn't resolved to cure himself of all of his illnesses, there would be no other supporters or helpers coming in from the outside to help them. They're on their own (with the exception of Dickon, of course), and the big, empty space of Misselthwaite Manor helps to underline this isolation.
We also talk a lot about the various settings of The Secret Garden elsewhere in this learning guide, so please excuse us while we send you off to other sections: We talk about the garden itself in the "Symbols" section, and we talk about India and England over in "Themes." Definitely check those out if you want to know more.