Quite few things.
First of all, and probably the most important of em all - you start behind tech-wise and it'll take awhile before you can catch up. Until somewhere in the Renaissance give or take, earlier with Babylon though. Don't let it scare you. It's more than normal. After you catch up the game becomes much easier and not so different from emperor.
Second, of course, the wonders. AI hits all techs earlier and thus you have a very little shot at them. Pick those you want to grab, plan ahead and go hard after them. With Babylon I'd definitely give a try at Pisa, however, depends on your opponents, it might go very early, so it's not a done deal. PT is more reliable, but again, you need to beeline it. Hubble is pretty secured if you hit Rocketry in reasonable timeframe and bulb Satellites with Oxford (and have engineer, obviously). Out of early ones I find ToA to be the best for science games. Half of the time AI overlooks it, and you have a decent chance to build it somewhere between 35-38. So with good productive capital, I'd give it a shot too probably. The rest are either not worth the hammers or not obtainable. Forget about GL. Really. Sometimes (like maybe 15% of the time) you can get it on ~40, but why would you want it as Babylon to begin with? The case would be much stronger with Korea, with any other civ just skip it.
Military-wise AI has tons of troops, although pretty crappy ones. It's too eager to spend the cash on purchasing new units, and leaves almost nothing for upgrades. So you'll see some carpeting with junk. Yet the bonuses are overwhelming and you need to be prepared. Personally I have no trouble to defend with archers early on, and usually don't rush CB's, unless I'm planning to go offensive. However, when pikes start appearing, you do need to upgrade. And yes, settle on hills. It's never bad.
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In the late game AI will have literally dozens of planes. 40-50 for a runaway is not uncommon. So don't neglect AA. And that's another difference. From mid-game on you can see AI with some cash and ability to pay their share of RA's and full price lux deals. Which, I guess, would be nice and refreshing change from your previous experience.
Also don't expect AI to stay small and tall. I've seen AI going OCC more than once on emperor. Not gonna happen on immortal. Expansion is quick and massive. Pay AI to go to war. As often as you can. Wars slow it down, now more than ever. And gun for pre-300 finish. You are not safe after this mark. BTW, both of your neighbors have a tendency to become runaways, so bribe them to DoW each other and join the party yourself. Trimming them down early sounds like a good idea to me.
Good luck!
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