How about this?
Make it so that anybody who wants to join the Trade League has to supply the steel, bricks, etc. for their own "Entrance" to the main market.
They port from their town into the small chamber on the right, access an alt that has their key for their gate to pass to their second crossroad, which leads to the main crossroad in the marketplace - the gated section for the main market could be a fail safe. If there's no trouble, keep it unlocked and open. If there's a threat brewing, close down the market till it blows over/is sorted.
Edit: The beauty is, hopefully, we won't need to seal the "Chamber" wall corners, so we could just extend into a grid fashion to save on steel. I could probably make the chambers smaller too for brick consumption.

Grand Walker of World 1 (w1).
Ex-Lawspeaker of New London, Deadwood and New Deadwood (w3).
Witness of the World 4 Great Explosion (w4).
Ex-Lawspeaker of Gregminster (w5).
Ex-Lawspeaker of Laketown (w6).
Squatter and Rebuilder of Templus Pacis (w6).
Hermit thereafter.
Lawspeaker of Tachbrookshire (Hafen 2)