The wiki says, under Purge Difficulty, that what mobs you get and how hard they hit is entirely determined by where your base is:
Desert and Savannah: tier 1
Swamp, Highlands, and Tundra: tier 2
Snow and Volcano: tier 3
And yet we had a guy on (PvE offical) server 1522 with a base in the eastern savannah, just northwest of al Merrayah, holler for help (and we did help him) because the first wave of skeleton warriors tore right through one of his tier 3 walls in seconds. And it’s a good thing we did: wave 4 was a legendary world boss, the Undead Dragon.
In the meantime, my base in the Highlands (just southwest of the Ruins of Velstad), also made of tier 3 materials, only gets puny waves of tier 2-3 Cimmerian warriors – no weapons or special attacks that can seriously damage structures, half the hit points of those skeleton warriors, no armor at all.
My first thought is that it’s just that buggy; that he shouldn’t be seeing skeleton warriors and I should be seeing a third and a fourth wave including (probably) a Rotbranch world boss. (In which case thank Derketo I’m not, because an offline purge with a legendary world boss is nuts.)
But I wonder if it is working as intended and there’s just something left out of the description. Because everybody I know who’s been hit that hard has a much, much bigger base than mine. Mine’s a single 4x7 single-story house with two tiny outbuildings; his was a something like 20x20 multi-story fortress. Is Purge Difficulty also dependent on the size of the base?