There is no single answer to this question across time, but I would point out a few things that jump out to me while thinking about it.

One is that the people who move into a different place tend to have reasons to be there. They are diplomats, ambassadors, trade representatives, what have you. These people are always the "anchors" of a spy network, and still are today. They recruit locals, if they can, to provide the more intimate information, since they are going to be conspicuously foreign.

In terms of communication, you have to remember that throughout human history, people have circulated. They trade. They fish. They move around. There are lots of "legitimate" means by which people in different countries communicated with each other. There are exceptions — such as when the Japanese were totally isolated from the world — but these are rare.

Lastly, the most effective and reliable way to communicate has not been through some kind of high tech or conspicuous means. Those are actually pretty easy to track. (The British got very good at finding Nazi spies within their ranks by tracking their short-wave radio transmissions.) The most effective way is through pre-existing communication channels, like diplomatic pouch or even just the postal system (where communications can be encrypted or have hidden messages in them — steganography) or wares being shipped from one place to another. Trying to create a totally new "channel" for communication is very conspicuous compared to using an existing one.

So I would think over this "world" of yours. Does it have trade of any kind, whether formal or informal? Are there people who would be circulating between these lands officially or unofficially? If so, that is your easiest means of circulating information easily and reliably, and could add additional depth to the story.

Hi there - we're happy to approve your question related to your creative project, and we are happy for people to answer. However, we should warn you that many flairs have become reluctant to answer questions for aspiring novelists and the like, based on past experience: some people working on creative projects have a tendency to try to pump historians for trivia while ignoring the bigger points they were making, while others have a tendency to argue with historians when the historical reality does not line up with what's needed for a particular scene or characterization. Please respect the answers of people who have generously given you their time, even if it's not always what you want to hear.

Additionally, as amazing as our flair panel is, we should also point out that /r/AskHistorians is not a professional historical consultation service. If you're asking a question here because you need vital research for a future commercial product such as a historical novel, you may be better off engaging a historical consultant at a fair hourly rate to answer these questions for you. We don't know what the going rate for consultancy work would be in your locality, but it may be worth looking into that if you have in-depth or highly plot-reliant questions for this project. Some /r/AskHistorians flairs could be receptive to working as a consultant in this way. However, if you wish for a flair here to do this work for you, you will need to organize this with them yourselves.

For more general advice about doing research to inform a creative project, please check out our Monday Methods post on the subject.