The idea of caravans between towns that have roads between them is very nice.
This is what I imagine it might work like.
1) The road is built between the towns, and a couple wagons are built by each town.
2) The town mayors get together and discuss trade goods. Mayor1 wants iron and copper ingots. Mayor2 wants leather and charcoal.
3) The agreement is signed for a certain exchange rate and priority, and players start collecting materials and depositing them in trading posts.
4) the caravans start travelling back and forth, trading as much as possible each time they get to one trading post or the other, based on mass, volume, value, and priority.
5) The roads are guarded based on the value of trade between the two towns, by anything from shephard boys to hardened companies of mercenaries. These guards might either roam the roads and nearby areas, or be located in player-built waystations. Perhaps both.