Gods & Kings includes 27 new units, 13 new buildings, nine new wonders, two new eras, and nine new playable civilizations. A new religion mechanic featuring a "faith" resource allows players to found their own religion... Diplomacy has been reworked to include espionage, foreign embassies, and new types of city-states... Additionally, the combat system has been re-worked to include smarter AI, an expanded early modern era, and enhanced naval combat... (wikipedia.org)