Trade square with high valuable stuff... like jewelry 500+, gold, silver, HQ metals and etc.
Because of high value, access there will have citys with 100% active people, good defence (brickwall is minimum) and of course good reputation.
At this point I see there AD, Sodom, Buyan, Angkor.
Crossroad to this trade place must be nonpublic. Also we thinking about use of charter stone, for use by citys that cannot have good defence but have persons with high trust level. Also 2 levels of brickwall with trade square in center and space between them to prevent any hunts by rangers.
I see there 2 type of stands:
Barter - selling silver coins for any other stuff.
Trade - selling products for silver coins.
I think silver coins will be the best thing, because I can set there 20 bars in silver coins to trade for gold, if you say 50 bars needed I can supply them for sale.
Also there can be set barter stands to change gold-silver in bars and nuggets, in case some materials needed fast.
This thing will allow faster trade between citys, and possibility to help each other in progress. Because each city have its special products that better than others... And for example sometimes you are ready to pay a LOT for that 1 piece of HQ silk, that you lack to finish your megatable right now, or you need that 1 nugget of hq steel to get your helmet done...

New candidates for trade in this place must be approved by all existing after check in secuirity. I hope Sodom and Buyan to help us with defence of outer brickwall to give 24/7 assurance this place will keep safe.