What do the 3+ and 4+ markings on cards mean, and how do you set up a 2-player game?

Question

Accepted Answer

Cards marked 3+ are used only with three or more players, and 4+ cards only with four players, so remove the ones above your player count (and the pact cards from the military decks in a 2-player game). Each of Civil Decks I to III has five 3+ and five 4+ cards. All 13 cards of the initial row are always placed.

Through the Ages: A Story of Civilization

What do the 3+ and 4+ markings on cards mean, and how do you set up a 2-player game?