at this point i am in second or third place on noble difficulty and am basically defenseless.
my main goal is to be far superior technology-wise at this point.
part of being superior technologically is staying alive of course! i often play scarily defenseless, but when i do that paying attention to diplomacy is
critical!
if you're getting along with your neighbors, it can give you time to build up a defense, and give you more trading opportunities. the importance of diplomacy (my pet phrase for it lately is jedi mind-tricks) really jumps as you jump in level. when you're learning at settler level, it matters somewhat (everything does), but you have so many advantages that it doesn't usually make or break the game. deity, well i only try that OCC so by default i
have to have at least one friend, the most i can produce is one troop per turn so if 7 civs are fighting me at once i'm a goner!
when noble (or any level) is a step up for you, and you're learning how to deal with it, i recommend paying more attention to the foreign advisor than you're used to. you might have to give into demands more often, etc. by "leveraging diplomacy" i mean managing it the way you want to, sometimes it's just paying attention. even in an Always War game where you know they'll never like you. you don't have to have any friends if you want to hold a beat'em'up fest. knowing who's in the "we love the aztecs fan club" and thus they're pretty easy for monsterzuma to bribe in and dogpile you is handy.
my preference is to play as peacefully as possible. i'm not good at beating folks up. i love to have attack puppies to help me when i do fight wars, etc. so i leverage the mind-tricks more than most do, and definitely more than most
have to. but seriously, i think learning how to finesse that stuff helps on any level, the higher the level the more crucial it is. even if you're not not facing a case of everybody and their brother blowing war horns at you, good relations lets you get away with more resource trades (bigger better cities wheeeeee) and tech trades. at Friendly they never even keep track of "we fear you are becoming too advanced" which is spiffy.
it's a pain to focus on that and everything else at the same time. but as you get used to the other stuff that's new (how fast you have to expand, how quickly will they tech, what techs are more important for you to get earlier at this level, etc), i'll bet you 2 golden ages that the better you get at that side of the game, it will help with whatever type of victory you're aiming for.
sorry this got so long! good luck and have fun