Re. "creatures only": Do you have a reference for that? I assume it's "creatures and possessions" and not "Terminator"-style naked?
Well, the premise is that the BBOG is aware and paranoid of magical overthrow and does everything in its power to discover and "re-educate" or kill casters. They have a Stasi-like "Magisterium of Arcana" with a monopoly on spell books and rare components, informers and spies, etc. The party time-traveled back to this era and even the 6th-level Cleric, Warlock, and Sorcerer (in particular) would be considered major threats and are being pursued by a Hans Landa-esque character who knows they exist, but no details. (Kind of off-topic, but it's been a really fun couple of sessions at the table developing this... :-) )
MacOS primarily