How does Trade Treaty work?

One thing to know about trade caravans - they provide sight into the area around them, so they act like free scouts. Monsters at least used to eat them, I don't know if they still do because I rarely find the AI to be trustworthy enough that I'd bother making treaties with them, but that's just because I'd as soon conquer them before they decide that they should conquer me as let them live in peace while we both build up strength for a later war, at least if they are neighbors of mine. It also serves as a pathway directly to your capitol, and to the capitol of your trade partner, so if you wanted to start a fast war to take over the (probably) best city in your trading partner's empire, this is one way to find that city. Also works the other way, and can be really annoying if it happens to run a road through the territory of that really aggressive faction nearby, or through the wildlands.

I think if you look at the breakdown of your money income, you'll see an entry referring to trade, and that will tell you what you gain from trading. Although that might have been from an economic treaty. If trade caravans are like they were in War of Magic, then they provide a percentage bonus to the income of the city served by the trade route, in which case you should be able to see it in city details (this is also probably where any direct income bonuses from trade caravans show up). If it works like trade in Galactic Civilizations II, then the trade bonus is an income bonus of X gold per turn.

One thing that is annoying though is that when you use the shortcut to get to the next unit (end key by default i believe) your caravans are part of the unit list ... Although I haven't checked if they changed that in the patch.

the trade treaty seems to give your capital a 10% boost in gildar production. You can see the effect in the tooltip on the capital city screen. As a bonus, if you conquer enemy capitals, those cities will retain the 10% bonuses from trade treaties they negotiated as well.

They still do.

i noticed that if you click on a caravan, it says something like:

current road level: trail

19 trips to next road level

and there's a little progress bar. what is this all about ? do caravans somehow upgrade your roads? given the rate that they get eaten by ogres, and the average length of a "trip", i'm not sure if i've ever even seen the second road level in a 300 turn game

Sounds like that may still be left over from WoM or early beta. Used to be, you built a caravan and sent it to a town. Then as that caravan made more and more trips, the trail/road got darker and darker. Not sure if there were bonuses for different road types or anything though.

When your caravan completes the required number of trips to the other capital city you will get a larger gold bonus in your capital. IE 10% bonus gold becomes 14%. The required number of trips is very high and they get killed a lot during wartime so you'll only see this when the other capital is very close to yours in a long game.

The gold bonus is correct - it starts at 10%, and the level progress will increase the gold bonus.  For this reason, I almost exclusively make my capital a town to capitalize on this bonus.

And yeah, I had a road at 14% once when Magnar was very close, but it didn't last long since they got wiped out pretty early .  At turn 350 or so in the same game, I have yet to get another trail upgraded to 14% gold - all treaties still only give a 10% bonus per treaty in my capital.

Do you keep the bonus if you take over someone else's capital? Ie. do you still get the caravan traveling between your capital and the newly captured one?

if this mechanic is going to be kept then, it needs to be buffed drastically. A number of total trips seems an unfair mechanism, since distances between cities can vary widely. perhaps instead, an xp that increments each turn the trade route is active (and maybe some xp is lost if a caravan is destroyed)

Don't most 4x games reward trade routes for being longer, not shorter?