Has there been a secret AI patch, or does the AI (as I've sometimes suspected) have a learning algorithm? In my last full game of Civ, my citadel was a lot less effective than usual at killing enemies - but it acted as a strong deterrent. This is because Nebuchadnezzar would actually move units away from it at the end of turn, and even exchange wounded units for intact ones. I've never known the AI play that way; its usual response to a citadel is to surround it and fortify, while rapidly dying, then rinse and repeat. What's more attacks from both Rome and Babylon were coordinated - both with each other and, if one power attacked, it did so with a big army consistently, rather than with a big army, losing it, and then throwing new units at me one by one as they were produced.
If only Neb hadn't been sitting with a whole bunch of fortified artillery within range of my units, and never actually activating them to fire, I could almost have thought I had a human opponent (albeit a not especially good one).
I just had a game today where Japan attacked me and he wisely retreated instead of coming forward like the AI usually does to get massacred. He fought a limited offensive, but it seemed like he could not get his troops into an advantageous position against me, so he withdrew back around Osaka. Its amazing, as if he could tell I was in superior terrain, which I was. Still, I could not believe my eyes.

I was so sure I had him. It was a foregone conclusion he was going to declare war. All he had to do was advance into my trap, like the AI usually does.
Also about a week ago, I decided to play on emperor for the first time. The first time I tried I lost my capital, because Japan and Russia ganged up on me and they coordinated their offensive against my capital. I tried and tried to regain my capital and never could. I was at one point down to one city, in which I had built a citadel to defend myself against Germany who backstabbed me. I was able to knock of some of his units, but then he simply retreated away from it. Interestingly enough, he was quite aggressive from a city in the south, which kept bringing troops to attack army from behind, which I had there trying to regain my capital from Japan. The AI seems like it has learned something. It made me feel stupid for a minute there. I gave up the game, and started a the one I described above, so far so good in that one. So it could be a preliminary secret patch. Although the only update I have noticed was for Fall of the Samurai on steam. I will let you know on any further AI developments as I play more.