How big must an empty area of map be to allow barbarian camps/villages to start reappearing? I had a crazy idea of leaving such a void area inside the Mayan empire and then farming slaves from it but then it occurred to me this might precipitate wars with other Civs crossing my land to get to it and set up home there. Also, one would have to be careful not to build any culture in the twins surrounding it in order to preserve its purity, like the Amazon rain forest or Antarctica or something. Still, I wondered if anyone knew.
Don't quote me on this, but I
believe that Barb-camps appear in previously explored/exposed squares which are currently beyond visual range both from the nearest cultural border(s), and also from
any unit, including AI-units, i.e. from your PoV, the squares where you can see the underlying terrain, but which are currently slightly greyed-out. If I'm right, and if you're lucky enough to roll a map which allows you to create a culture-free hole within your own borders, then your barb-farming idea should work.
How big a hole?
A 1x1 culture-hole (cities at CxVxC, where V=visible tile, nominally beyond borders) will usually get closed automatically
A 2x2 hole (cities at CxVVxC) might not automatically close at CityCulture=0, but all the tiles within it would be visible from your borders
A 3x3 hole (cities at CxVRVxC, where R=non-visible reserve) might not be very useful for barb-breeding -- because unless that was the last free non-visible tile on the map, you might not get many camps spawning there
A 4x4 hole (cities at CxVRRVxC), would give a non-visible barb-reservation of 2x2 tiles
So the smallest useful hole would have to be at least 3x3; you'd also need to make sure that all those tiles were roaded, so you could get your units in and out in no more than 1-2T. For a reserve any larger than that, you'd probably want a Large or Huge map, so as to minimise the percentage of land that you're trying to hold as a reserve, and thus can't use productively.
And you would have to avoid signing RoPs -- at least, with the AICivs on your continent -- or they
would try and plonk a town(s) in that 3x3 hole. Also, because the AI
also loves going barb-hunting, you'd want to be sure that you could attack and hopefully enSlave any spawned barb before the AI-units could reach it. Without an RoP, incoming AI-units might indicate that a camp had appeared on your reserve. Once you've dispersed the camp, hopefully enSlaved its defender, and retreated your units, the AI barb-hunters should also lose interest, and retreat back beyond your borders. You'd probably want to avoid having to boot them, though, since that would risk them either getting to the barb-camp quicker, or DoWing you.
One possibility that would alleviate both the 'lost land' and the 'AI-Settler' problems (although not the 'AI barb-hunter' problem), would be to create your barb-reservation within a mountainous region on a 3-bn year map. That way, you might even be able to use a 2x2 reserve, if the cities are built with high ground between them and the reserve.
All in all though, seems like a lot of trouble to go to when you could just enSlave the AI-citizens and units instead. Or were you hoping to use this idea to obtain lots of early barb-Slaves without triggering your GA?