Winthor
6
It still works if they do not have anything to spawn on. Means any bigger flat areas that are not occupied with benches, decor or other stuff that blocks the tiles are a potential target. Especially the roof.
You can controll the purge if you give them something to go for. Build a sandstone plattform outside where they can spawn or walk on. Purges love sandstone and flat areas. If you have big unused space inside your buildings try to lay down carpets and do some decorations here ad there to block the area. And do a proper roof, that flat area is crying for a purge to spawn on.