Very good idea, you need to have a gameplay idea about it.
De gueules à la tour d'argent ouverte, crénelée de trois pièces, sommée d'un donjon ajouré, crénelé de deux pièces Gules an open tower silver, crenellated three parts, topped by a apertured turret, crenellated two parts
You could have it like the Forbidden City with several Cardinals from around the world on the outside trying to get in to become Pope; or even the artists like Michealangelo and Brumante trying to get tough it out for the favours of the Pope. Research would be very easy. I would take up the challenge as ST Peters has always fascinated me, but i have some other maps to finish first.
* Pearl Harbour * Waterloo * Forbidden City * Jamaica * Pot Mosbi