AI CD, AI HD, AND AI???

I'm a bit confused about how the AI settings work.

Just had to step away from the PC because i got a little worked up.

I started a game as the Mayans, in Yucatan, with 3 other opponents, free for all.

I was up against Britons, Celts, and Franks. I found myself at a loss, I never got so cornered before. All 3 were constantly attacking me, wave after wave, they kept sending more troops and siege weapons. I was fighting the franks and almost had them, but the Britons came in and decimated what small forces i had left. I had to return to my settlement when I find the Celts had broken through my walls and were destroying my castles. I resigned and watched for a bit, to notice neither of them were fighting each other, and they were all attacking me. Even when I resigned they still kept attacking my settlement. I don't know what happened but had to vent a bit and ask a question about the AI settings. I only found small answers about the AI settings on google. and most YouTube vids only show AI vs AI game plays.(that and I'm too impatient atm to watch a 34 minute vid) How do the AI settings work? which is supposed to be more challenging and which is more casual?