In Saint Petersburg you'll be buying different types of cards: workers, buildings, aristocrats, trading cards and the new market cards. In every phase a new type of card will be available for purchase. You'll start with workers, who are good at bringing in rubles. The new market (yellow) cards, bring in new resources. Having the majorities in these resources from round to round will earn you additional victory points. This battle for the lead enhances the game experience, and expands the tactical and strategic possibilities of the players. In the next phase buildings can be acquired, mostly granting victory points. Aristocrats provide a mix of victory points and rubles, and also lead to a large end-game point bonus that can't be ignored.
Trading cards allow already purchased cards to be improved. These phases will repeat until a one of the card decks runs out. At this point the final victory points are counted, and the winner is declared.