I'm going to run a tabletop RPG campaign based in the 1540s and I want to make it as historically accurate (except for the addition of magic) as I possibly can. Another friend recently ran a similar one in 1920 and we had to retcon half the session because he added several historical inaccuracies so the players argued he was making things up to artificially increase difficulty. I want to know enough to avoid ruining immersion and to keep my setting somewhat historically accurate
I'm afraid I'm going to need more specific questions in order to give you a satisfactory answer... anything in particular you wanted to ask with regard to Rome and/or the rest of the Papal States? Language? Society? Architecture?
A reply to /u/Daxxacar
1540s in the Papal States? Oh boy. So many things.
That's not even half of a full list I can think of in 2 minutes.
With no indication of how much you know and how willing you are to learn, this is impossible to answer fairly, just as /u/Quirite said.