Gods & Kings includes 27 new units, 13 new buildings, nine new wonders and nine new playable civilizations. A new religion mechanic featuring a "faith" resource allows players to found their own religion which they can grow into a "world-spanning fully customized religion." Diplomacy has been reworked to include espionage, foreign embassies, and new types of city-states (religious and mercantile). City-states also feature a new quest system and strategic importance. Additionally, the combat system has been re-worked to include smarter AI, an expanded early modern era, and enhanced naval combat featuring Great Admiral and melee naval units. Gods & Kings adds an additional 52 Steam achievements to Civilization V.