I played two games all the way through on normal. I then played one on hard, and I just played one on impossible.
I didn't think hard was all that much harder than normal at all. I didn't get a good start. I could only build three cities because I was pressed up against a couple of neutral cities. I was kind of worried at the beginning while I was building up, but when I got done I exploded and wiped out players 1, 2, 3, 4.
The game on impossible gave me a much better start. I had room to build six cities, but I'd barely begun when the nearest guy started walking stacks into my territory. I was just about to finish the forrester's guild in my capital when he declared war. He already had a stack of units right next to my capital that could wipe out my entire empire. I'd known this was a lost cause for some time. He was just ridiculously more developed than I was.
So, my impression is that the AI on impossible is just as stupid and disorganized as it is at lower difficulties. It just throws a lot of cheating power at you. But apparently it is possible to win at this difficulty. Obviously, it's all about speed. You don't have time to build forrester's guilds. Or do you just do this with the land setting to small and hope you get a decent-sized island with no enemy presence?