You choose to be one of five feuding lords vying for the Crown. Conquering, spying, sabotage, diplomacy, whatever it takes--as long as you keep the Pope and your people happy. You have to reach 7,000 points to petition the Pope for the throne, and then you have to maintain those points for 4-5 game months to win the game. However, your rivals will attack you as soon as the Pope declares his support, so be sure you have a strong military before applying for the Papal backing.
