I'm surprised only two people have mentioned religion, and that only as an aside. To start off with, religion is at its most powerful with a wide empire. That's because religious costs don't scale but income (faith) and benefits do scale (the more cities you have, the more shrines, temples, pagodas etc.). That is, it's not like culture where costs go up the more the more cities you have. When going wide, prioritize an early religion, maybe build Oracle, and look for the following: Goddess of Love (+1 / city of size 6) Ceremonial Burial (+1 / 2 cities in any civ) Asceticism (+1 / shrine) Cathedrals (+1 / city) Mosques (+1 / city) Pagodas (+2 / city) Peace Gardens (+2 / garden) Religious Center (+2 / temple) If you prioritize and thus get an enhanced religion before the AIs take the above, it's typical to get an easy +4 or +5 per city. That's huge and critical to a wide empire.