The AI gets more agressive and opportunistic. But in the end, you won't get attacked with stacks full of veteran upgraded armies with great generals and stuff. Cities are stacked with veteran upgraded armies with great generals though, so bring your own great general and find a hill to unleash your stacks from.
The AI comes at you with one or two armies and sometimes even single units. But still, you need to be properly defended or you're toast.
I'm a prince player with Civ 4 (level 5 of 9), but I can win with a proper challenge on Deity with Civ Rev.