A key inspiration for City and Guild was the London Underground map. I saw a documentary on it years ago, and it talked about the difficulty in representing junctions. That gave me the idea for the maps junctions for traders, and these were ridiculously important for getting the trade system to work. Here’s why:
At its simplest the trade systems distances are written up using a website that compares the GPS coordinates of two towns andgives a distance from one to another. This seems simple, but its doesn’t scale. It’s fine to measure this way if you have three cities, becasue an extra city only costs you another 3 measurements. The tables in City and Guild, though, have over 50 cities on them. You can’t measure over 50 other distances, then check the routes for rivers and other terrain features, just to add a series of progressively smaller towns.
The point about the junctions at Brittany, Gibraltar, Sicily and Skagarek is that they break this geometric progression, or at least tame it. Since all traffic between the Atlantic and Mediterranean must pass by Gibraltar, there’s no need to measure the distance from an Atlantic port to each of the Med ports, or vice versa. You just measure to the junction.