On Towards the Stars: Trade
[This post is part of a larger strategy guide for Civilization 6]
It's tempting to ignore trade and trade routes the first time you play civ. Unlike Campuses or Theater Squares, Commercial Hubs aren't tied to a particular victory condition, so they don't feel important. But I think it's basically impossible to beat even an emperor AI — much less another human — without a strong trade network. In the early game, domestic trade routes are critical for bringing food and production to new cities, kick starting their ramp. In the late game, international trade routes are often the largest source of gold, as well as a hefty chunk of science and culture. In most cities, in most games, the commercial hub/market (or harbor/lighthouse) are some of the first things I build.
So with all that said, some tactical basics on trade routes.
Early game, send your trade routes from your newest cities to the one that gives you the most bonuses to food/production (generally the capital). The one exception is if a city state will give you an envoy for a trade route quest.
If you're preparing for an aggressive war, send a trade route to your enemy's city a few turns before you declare so your reinforcement units can move to the front faster. This is especially needed if there are a lot of hills or woods in your way.
If you're building a wonder in a city, move your trade routes to start from that city so that you can get more production.
Trade routes give a fixed amount of resources per turn based on when you start the route. But as the game goes on — and cities get bigger and add more districts and so on — the amount of resources per turn that you can get from a route goes up. That means you want to restart your routes as frequently as possible. Trade routes have to end in the origin city AND have to go for at least a certain minimum amount of time. That means you need to be careful about how far your routes are. Let's say a trade route has to be out for at least 13 turns. If you choose a city that's 6 tiles away, your trade route will do a loop over 12 turns. Since it hasn't hit the 13 turn limit, it will go back and do the loop again, effectively wasting 12 turns where you could've reset the route and earned more from it. So you want to be planning your route distances based on how long they need to run. Now, actually doing all this math will drive you insane, but luckily the civ fandom wiki has a super convenient table for optimal trade route lengths.
Trade routes get big coin bonuses for going through tunnels, canals, railroads, or even over water. The bonus is calculated as a ratio of some magic efficiency number and the total length. Done correctly, and you can double your gold output for your routes. Again, the civ wiki has all the math, but when you have a choice you should try and send your trade routes along one or more of these special features.
If you're going for a culture victory, you want to have trade routes running to as many other civs as possible. Routes increase your tourism output to that city, and the build up over time matters a lot.
Finally, there are a lot of civs that have unique bonuses that depend on trade routes. If you're Portugal or Tokugawa, for eg, trade routes are basically the whole game. I'm not going to dive super deep into all of that here, but I'll leave off by saying that trade is important enough to get it's own little chapter. It's very worth getting familiar with the mechanic.