However, I'm always wary of using costal squares at the expense of cottages. You get an early gold rush but they can't grow to the same level of commerce as a cottage can.
Well, to grow a science powerhouse or GP farm, one obviously does not want to go coastal with such a city. Still, the commercial limit of coastal tiles (4 with Colossus when financial) is partly compensated for by (usually) better trade routes in coastal cities (and also more, if you have the GL). This holds even more true if you are alone on your continent -- after Astronomy, the coastal cities produce a lot more trade commerce than the continental cities. So, the trade routes partly compensate for the lack of cottages (in varying degrees, depending on wonders, buildings, civics and geographic situation). I also like that the fact that you have to worry less about all this commerce (it can't be pillaged and doesn't need much management). Keeps my limited brain free for other tasks (like micromanaging my whipping

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Unless you beeline to writing you can set up an enclave well before you actually have the opportunity to open borders with other civs. [...]
The purpose of setting up a couple of barrier cities is not so much to grab all of the land between the border cities and your capital but to have first choice of all of the best sites in the area without fear of being beaten to them.
The point of my question was something else: The strategy of "boxing in" an opponent via land grab and fast border expansion (creative trait), does it work well if you have open borders with that opponent? The AI might take into account higher maintenance and strategic disadvantages when evaluating possible city spots, or it might not and simply march into your "claim" and found an exclave.
i think, it's because lots people usually stay at prince
indeed the higher the difficulty, the more expansive traits becomes useful
the fact that barbarian are AI in higher difficulties are way more annoying (killing cottages), make it very hard to run a CE
running SE = you need lots people = you need health bonus
OK, as I've said before, I'm not really fond of the idea of introducing artificial, and unbalancing, modifications to the rules to achieve higher difficulties. Of course, because the Civ4 AI is quite weak, you have no other choice if you want the game to be really hard. I'm playing only Blake's Better AI now, usually on Prince (which roughly equals Monarch with the vanilla AI), because that way I can stay closer to the "actual rules" (Noble) and still enjoy a challenge (BTW I highly recommend Better AI).
My line of thought is the following: if a given trait (Expansive in this case) gains a lot of value in the presence of artificial rule tweaks (difficulty level), to the extent that this becomes a major argument in favor of this trait, then something is wrong, because the rule tweaks shouldn't be there in the first place (given a strong enough AI they don't have to).
Anyway, I get the point about the value of being Expansive when running a specialist economy and I clearly see why Peter (Exp/Phi) is an excellent leader for such an approach (indeed I'm planning to improve my specialist handling, and you can be sure I'm gonna choose Peter for that

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--Sigi