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It depends on your platform. We use Unity's PlayerPrefs system to store data.
On Windows it's in your registry (under "HKCU\Software\TeagherStudio\Just More Doors").
On Mac it's stored here -> "~/Library/Preferences/com.TeagherStudio.Just More Doors.plist" (or something like that, never tried it on Mac).
On Linux it's here -> "~/.config/unity3d/TeagherStudio/Just More Doors".