Ok so slowly figuring this out.... I see in the City Details window, that if I create an economic treaty... is show me I have Caravans with the other factions city.... it looks to me like it provides flat +10% gildar bonus?

Isn't that what the trade treaty does.... (is there any difference other than the roads) ?

It multiplies your guildar per turn by 1.10.  I believe it also improves your relations with the other faction, don't quote me on that. It's generally a good idea to make as many of these treaties as you can. 

new gildar income = gildar * 1.10 

Yeah just worked out it is only the capital.

Is there any way to get more caravans....?  You're stuck with 10% for each economic treaty right?

You can attack caravans, but they are just a nuisance.

in older versions the ai spammed world spells on them wasting their mana, its supposed to be fixed but i haven't really paid attention to them.