On a much less complicated note, what if Goblins sent ambushes through caverns?
Natural caverns? I'd just assume that enemies would use any open pathing available to them. So yes to that. There's no good reason, even player-driven, to disallow that.
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And, while it's absolutely
not my style of play, I still think it's important to allow for the player who simply enjoys building castles and other such projects, to do that in Dwarf Fortress.
Even if all they want to do is build
sand castles.
Different people enjoy the game, in lots of different ways, for lots of different reasons. That's a good thing. It makes for a broad player-base, and healthier donations.
There's no need for those of us who like the idea of seiges, and tunneling, and destruction, to inflict it on those who don't want those elements in their game. That attitude of
forcing tunneling, isn't any likelier to get us what we want, than just asking for it as a (main) option.
It isn't the same thing as adding Z levels, and
requiring every player to have those elements in their game would almost be a form of bullying, in a way.
I dislike bullying considerably more than I dislike people who are different than I am.
As much as I am for tunneling, we can all have our cake and eat it too, and everyone should be able to live with that.