Apparently trade routes are a seperate type of economy? They're in that little box on the left. Why is it with 10 cities, all connected, any singular city might just have 1 route, which goes to your capital, bringing in 2 gold... While other times, your capital might have 3 or 4 foreign cities in that box, each bringing in 4 or 5 gold.

Naturally the multipliers make sense. Harbor, Artemis, etc. But what's a crash course in that? Thanks.

I've paraphrased this from another (pre-Warlords) thread, so apologies if it sounds a bit too simplistic ...

City trade routes; http://forums.civfanatics.com/images/smilies/civ4/traderoute2.gif, add extra Commerce; :commerce: to their output, which may be converted into Gold (Cash) :gold:, Science :science:, or Culture :culture:.

http://www.users.bigpond.com/c.hills/cumae.gif

In this case, Cumae has a two :commerce: trade route with Kumbi Saleh and is yielding one :commerce: from the city square. Because the commerce-slider is at 80% :gold: - 20% :science:, the three :commerce: are converted into three :gold:. You can add extra trade routes through buldings and Civics. The computer selects the trade route automatically.

You can have trade routes within your own empire ... in fact, the Mercantilism civic requires only trade routes within your own empire.

Nazrack* - You start off with one trade route to another known and connected city. You add more through things such as; The Great Lighthouse wonder, the Free Market civic, Currency, Corporation, Airport, and adoption of a 'Single Currency' through a United Nations vote.

Sailing enables trade routes along coasts, while Astronomy enables trade routes across oceans. Open borders are required.

Side note, you can only have
one trade route in your whole empire to each foreign city,
but you can have one trade route in each City to each of your own cities

So all 10 of your non-capital cities could have a trade route to your capital (if they are connected and didn't have a better option)

So why do some cities generate 2gpt and some 5gpt. What's the determining factor. Population? Distance?

Furthermore, sounds like any single city can only have about 4 trade routes. Best scenario: One regular, +2 great lighthouse, +1 cothon, for example. etc. maybe the castle, or such. Is making a trade route civ even worth it?

Formula is

Pop of city that Gets the commerce +10 +5 if this city is Capital

*

Minimum of
Pop of City the route is with
Distance to the City (this is multiplied by a factor depending on world size)

*2.5 if it is a foreign route

*1+50% if Harbor+100% if ToA

/200

Foreign routes give a Minimum of 2
Domestic routes give a Minuimum of 1

Trade routes possible
1-Regular
1-Currency tech
2-Great Lighthouse..Coastal only
1-Corporation tech (obsoletes Great Lighthouse)
1-Castle
1-Free Market Civic (requires Economics which obsoletes castle)
1-Airport
1-UN Resolution
1-cothon..Coastal Carthaginian only

So the number of routes non Carthaginians can have by Era [and a extreme guesstimate total yield per city, assuming decent number of trade partners]
Ancient 1 (3 with GL) [~2 5 GL]
Classical 2 (4 with GL) [~3 10 GL]
Middle Ages 3 (5 with GL) [~5 13 GL]
Renaissance 4 [~8]
Industrial 4 [~10]
Modern 6 [~15]

For how much a Foreign, non Capital/non-harbor route is worth, when both cities are X population (and sufficiently far apart) with No Harbor

12->3
14->4
16->5
18->6
20->7
21->8

Now that is with No Harbor, and not the Capital... they will never be a major source of commerce compared to Tiles, but they are free, tiles need to have a population working them

[edited error: Cothon thanks civdude, not thinking]

Also note: trade routes are one-direction only. New York may have a trade route to Shanghai, but that doesn't mean Shanghai has a trade route to New York.

Cothon is Cartharge's UB.

On Monarch, I actually use trade routes as my only source of commerce (I find cottages a complete waste of an improvement). If you build the great lighthouse and build a primarily coastal empire and maintain open borders, you will find that even a size 2 city is generating 4-12ish commerce (a harbor in every city is essential). Temple of Artemis in your capital doesn't hurt either, as each of your 4-5 trade routes will be making from about 8-13 commerce depending on the size of your capital. Which can easily mean 40-50 commerce in your capital just from trade routes. It's important though that you keep open borders and on the coast, or your economy will start to die. Also, it's important that you build primarily farms as improvements in your cities, as the extra growth will result in extra income. It may not be the best economy, but it keeps me ahead of the AIs on monarch, and it allows me to focus for growth and production in nearly all my cities.

-Bubiyuqn

^^interesting BUbiqyuqn, but what do you do when you go to war? Surely, that puts a significant hit on your economy?

The computer automatically reselects another firendly city of a different nation to trade with if a particular nation's cities close off due to war. If he goes to war with all civs he had open borders with, then he can no longer do foreign trade and is restricted to domestic routes. This is unlikely, though, and if he is suddenly at war with all civs, he has bigger problems than research anyway.

Tristan is absolutely correct. The best way to keep your trade routes up is to keep open borders with someone far away and coastal. That way, you'll just end up trading with them the whole game, which will eventually add a +2 modifier which makes it even easier to keep relations with them. If you go to war with your neighbors, your trade routes should be unaffected.

-Bubiyuqn