Hi Brennall! Let me see if I can give a simplified example that will address your questions.
March: Class VI. 320 families. Hides/furs -2, Grain -2, Spices +2, Silk +2.
Barony North: Class VI. 160 families. Hides/furs -1, Grain 0, Spices 0, Silk +1.
Barony East: Class VI. 160 families. Hides/furs +1, Grain +1, Spices -1, Silk -1.
Baron South: Class VI. 160 families. Hides/furs +2, Grain 0, Spices -2, Silk -2.
Barony West: Class VI. 160 families. Hides/furs 0, Grain +2, Spices +1, Silk 0.
Let's assume that the Baronies form a + sign or cross, with each Barony sitting at one point of the cross. The March sits in the middle of the + sign.
Let's assume that the March has a trade route with all 4. Barony North trades with West and East. Barony West trades with North and South. Barony South trades with West and East. Barony East trades with North and South. Very straightforward.
The rules state: "When a trade route connects two markets, the smaller market has all of its demand modifiers shifted by 2 points closer to the larger market’s demand modifiers (or set equal to the larger market’s demand modifiers if separated by less than 2 points). If the two markets connected by the trade route are of equal size, each shifts each of its demand modifiers by 1 point closer to the other market’s demand modifiers. When shifting demand modifiers for a region, start with the largest market and work outward to its direct trade routes, and then from there to the next markets, and so on."
So, we are going to start with the March, because it's the largest Market (320 families). Since it's the same market class as the regions its trading with (Class VI), it and its trading partners each shift 1 point towards each other.
For March/North trade route: March gets +1 to Hides/furs, +1 to Grain, -1 to Spices, -1 to Silk. North gets -1 to Hides/furs, -1 to Grain, +1 to Spices, +1 to Silk.
For March/East: March gets +1 to Hides/furs, +1 to Grain, -1 to Spices, -1 to Silk. East gets -1 to Hides/furs, -1 to Grain, +1 to Spices, +1 to Silk.
For March/South: March gets +1 to Hides/furs, +1 to Grain, -1 to Spices, -1 to Silk. South gets -1 to Hides/furs, -1 to Grain, +1 to Spices, +1 to Silk.
For March/West, March gets +1 to Hides/furs, +1 to Grain, -1 to Spices, -1 to Silk. South gets -1 to Hides/furs, -1 to Grain, +1 to Spices, +1 to Silk.
Therefore, after accounting for the influence of March's trade routes, these are the new modifiers:
March: Class VI. 320 families. Hides/furs +2, Grain +2, Spices -2, Silk -2.
Barony North: Class VI. 160 families. Hides/furs -2, Grain -1, Spices +1, Silk +2.
Barony East: Class VI. 160 families. Hides/furs 0, Grain 0, Spices 0, Silk 0.
Baron South: Class VI. 160 families. Hides/furs +1, Grain -1, Spices -1, Silk -1.
Barony West: Class VI. 160 families. Hides/furs -1, Grain +1, Spices +2, Silk +1.
Now, what ends up making this a fairly complex exercise is because all of the markets are exactly the same size, so the trade routes are done simultaneously. That's a somewhat artificial situation, because usually the trade routes will be between at least somewhat differently sized towns, and will have been set up over time.
(more in next post)