Even on lower difficulties, the AI sometimes spams units. Turn 90, Japan comes strolling up to Orleans with 2 Catapults, 2 Archers, 4 Swordsmen and 2 Warriors with a Great General backup...on KING!
It just depends on which AI you're facing, how big the AI's need for war is (that is; how much does it like your lands) and how their economy is faring.