Playing with 8 AI on a small map - several hundred turns in.  I get lucky and manage to gather quite a few resources with the sea on one side, and the Yellow empire on the other.  Another sign of good luck is a giant mountainpass that seperates us - literally 2 places to invade from (both which I have capped with a city wall).  Yet yellow is a massive military might.  I get messages as it conquers players 1 by 1, going stronger and stronger.  It is probably 10x stronger militarily than I am, though an equal economy.  Brief attempts at scouting were put down by 400-combat armies.  I decide to turtle - closing 1 of the entrances with mountains so I can concentrate all defenses at a single crucial point.

It finally declares war on me, after letting me prepare for quite some time.  I expected to lose in the first battle - instead they sent their king with a few peasents (200~ combat) against my city of 900.  They lost, I won.  The whole war.

The AI sent their king on a suicidal mission to attack a frontline city.  When they lost their king their entire empire dissolved instantly.  Is this something that easy AI does often?