ik this is old but for anyone coming across this,

You can access the "outside" by enabling cheats (press = [equals] key). Since the outside isn't done, though, it will just be a map with some highwaymen you can interact with. Interestingly, this also gives you access to 2 more fort types that aren't used ingame to my knowledge

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It's kind of upsetting that this was perfectly possible in older version of the mod but has since been removed completely by the mod author.