Guild

A merchant guild is the oldest and most powerful kind of guild. Every city has merchants which bring in goods not produced locally. Trade itself is very old, older than cities themselves. The organization of the guild domain is probabaly dominated organizationally by the merchant guild. As a shorthand, one could think of a guild domain entirely as a merchant guild and hand-wave the existence of craft and resource guilds as simply assumed.
Socially, merchants rank above craft guild members, and are almost always wealthier. In many towns, government is mostly populated by leaders of the merchant guild. In some towns, their is no government per se, the merchant guild runs the town. Even in towns where the merchants are not in control, such as towns run by a lord, the merchants are very powerful and are consulted in the running of the town.
Merchant power is very great in towns and cities, but it is also much more confined to towns and cities. Outside of the town, the merchant must rely on their network of contacts and their money to achieve their will and guarantee their security.
Merchant guilds are involved in buying goods in one place and transporting them elsewhere to be sold. Making things and selling them to final users are the business of the craft guilds. Merchants are generally concerned with wholesale goods. Some will buy local goods from craft or resource guilds and transport these to known (or sometimes unknown) places far away to sell them at a higher price. Others go elsewhere, purchase goods and sell them at home. This raises the question of shipping guilds, but once can easily see that in a guild domain, there is a real value in bringing many guilds together for mutual benefit.
Merchant guilds are concerned with a higher level of order as wholesalers. Their interest lies in the success of others to produce resources or crafts, to find both primary resources and customers. As such, they are often closely involved in the rates of tariffs on foreign goods and the tolls on foreign merchants entering the city. Under this arrangement guild domains can be assumed to do business more widely than just the areas where they have holdings, but because they lack political controls to set prices, territories, tariffs, or tolls, these enterprises offer little more than a break even situation for the outsider, and can be a source of revenue for the guilders in control. Guilds with holdings can be presumed to have preferential access to the market, owning the best spots to sell or buy goods, owning facilities to make receiving, storing, or selling goods less expensive than one's competitors. Having legal or simply practical ways to avoid taxes or fees that outsiders cannot evade. If, for example, you own the docks, and there is a docking fee, as far as your guild is considered the right hand is paying the left hand. But foreign merchants are paying you for the pleasure of being your competitor.

An important part of any guild domain, craft guilds make what the merchant guilds sell. Unlike the merchant guilds, the craft guilds wield much less power in the town, even combined, because they have control only over the specific craft item they produce. Most regulation of the craft guilds concerns who may produce goods regulated by the guild, their quality, and their price.

An important part of the BIRTHRIGHT setting is the conflict between the exploitation of the land, and harm done to the land and its effect on Mebhaighl. Lumbering and mining is often mentioned as a destructive and pernicious form of guild activity.