Early trade zones

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I guess I'm not the first to ask about this, but I couldn't find any post about it:

At the beginning of the game, you have to get the "Sailing" tech in order to unlock the river and sea trade... But as many of you have noticed for sure, you can create trade zones using river or sea trade WITHOUT "Sailing", provided that the cities are "culturally connected", meaning that they are part of a continuous cultural zone.

The algorithm seems to be:

Two cities are trade-connected IF:

They are connected by road

OR

Sailing is available AND cities are connected by see or river

OR

cities are connected by see or river AND culture


Is it intentionnal or is it a bug?

I precise I'm using BtS 3.17, with the unofficial patch.

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[pigswill]

Its intentional.

 

[Supr49er]

That's how it's supposed to work.
Overseas trade routes are available after you discover Astronomy.

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