FREE Units: The AIs receive FREE units as soon as they build their first city on Monarch, Emperor, and Deity levels.
On Monarch level, the AI usually gets a free Warrior unit.
On Emperor level, the AI usually gets 2 or 3 free units. In Sman's experiment, the AI got 2 warriors and 1 archer.
On Deity level, the AI usually gets 4 or 5 free units, such as 2 archers, 1 spearman, and 1 worker.
The free units the AI gets appear to be random ancient units.
Greater chance of getting goodies when you visit a goody hut on lower levels.
More units require no support on Chieftain and Warlord.
Corruption level is higher for higher levels.
Easier diplomatic opponents on lower levels.