TRADE ROUTES link two cities with lasting commerce.
Build a CARAVAN, send it to another city, and establish a route for a one-time gold and science bonus.
The route then adds ongoing TRADE to both cities.
Routes pay best between distant, large cities — especially with a foreign civilization.
Only a city's best routes pay: one for every four citizens, so choose partners well.
A CARAVAN can also help build a wonder instead.