If it's just a peninsula blocked off by mountains...just go around! Sailing is a cheap tech that comes so early it's silly...and all you need is ONE galley...! Put a military unit or two on it as your settler/worker builds, shuttle them across, then go back and get the settler and worker. Add more workers/settlers/military as needed this way. Mountains don't change the fact that it's the same continent, so you won't be paying colony expenses or anything either.
If you are truly isolated, then you'll use minimal defenses while building up economy. Get cities up quickly, and if you have some good food resources whip out monuments/granaries/libraries and either cottage it up or do the SE if that's your thing (as darius, I'd go cottages). Of course if you aren't isolated, you might want an extra galley or two to move troops as they are produced, but you really don't need that many for short island hopping or mountain workarounds! Unless I'm invading straight into enemy borders (I'm one of the few people that seems to like doing this with galleys), 2 always seems enough for me for just shuttling units, easily.
You want optics fairly soon, but I don't usually recommend straight beelining it over currency/col. Often I run an engineer off the forge or use a GE generating wonder to get me one, then use it to bulb machinery, which is a major beaker-heavy tech en route to optics...doing this lets me tech along the top path without losing too much time.
Also useful after optics is getting guilds (which if you're going down the Lib path that this point you can usually just trade for), because then workshops equal mines when you're in caste, meaning you can actually make production cities anywhere that has food...very useful on otherwise hammer-poor starts.
Anyway, optics will let you explore the world and find everyone. Try to find everyone before tech trading so you don't piss the world off. I will say that on prince it's easily possible to stay in FRONT of the AI in techs despite isolation, as long as you get enough cities/manage them well. I can often manage a strong enough tech lead on monarch that I can invade with cannons/grenadiers while the AI is medieval or has only muskets to defend (which suck vs cannons), and sometimes even in the times of trebs/muskets! (I usually take astro off liberalism in isolation). I struggle a bit on emperor still but getting better.
Note that removing tech brokering should help you if you're in isolation, the AI's will be swapping significantly less techs that way.
Of course, if you're peaceful you can just sit there, pump culture in 3 cities, and specialize at least 1 or two of the others with workshops and spam defense constantly (no need for iron when pumping longbows and muskets etc). I'd still settle off the peninsula though...why not grab more good tiles and tech more quickly
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Get that galley and keep expanding
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