This is a sort of offshoot of the idea of extending the medieval paradigm and has to do with medieval and fantasy economics. Mainly, the issues here at play are those of modern capitalist style investment and the medieval economy; as modern people ourselves, we tend to struggle to imagine a world without capital investment or with substantially different economic means that are tangential in all ways to true "capitalism" (here I mean the movement of capital and investors rather than "a system of money" which I 've discovered some people think capitalism means). We're creatures of our own time and its difficult sometimes to understand the economic structures of different times. Just as we must struggle valiantly to comprehend cultural gaps, the gaps of economy are equally as hard to bridge.
It's easy to envision a capital based economy and I would argue that most people have a distinct vision of fantasy worlds (and even, perhaps, the middle ages) as functioning not dissimilarly to our own in terms of capital control. You simply replace the capitalist of the modern age (the banker, the investor) with a wealthy merchant or nobleman and viola! You have a fantasy paradigm! Or do you...? What you actually have is a shallow reskinning of the modern age.
Let us reiterate what capitalism is: the concentration of capital in the hands of a (relatively) small class of men (or women) and its use as funds to begin and own business ventures. However you slice it, that is not the way money or businesses worked in the middle ages. Capital investment was basically non-existant because large pools of capital weren't present. The middle ages functioned off of manorialism for the most part. This is the organization of rural economies in the villa system by which a lord profits off the labor of his land.
People didn't scrape around to find investors for their business in the middle ages, nor did they find a bank and take a loan. These practices did not exist and many things that we associate with them did not exist either. Essentially, the merchant class was one devoted to moving goods from market to market, not aggregating all manner of goods in any given place. The entire notion of a "business" would be falsely applied to any medieval operation. There were no general stores, feed stores, or hardware stores—there were people who produced each of these things (hammers, nails, food) and these individual craftsmen sold them from their homes.
So what are the methods of economic development that we can follow for a fantasy setting to give it more of that "authentic zest" I'm always talking about? Well, we can always rely on the Guilds, Costers, and Joint-Stock Companies. These present varying degrees of anachronism or fantasticism of the medieval economy, essentially in the order that they have been listed with Joint-Stock Companies reaching the farthest flung realm of anachronism.
What is a Guild?
An official, sanctioned guild (not a thieves guild, which is essentially the fantasy version of the Mob) is a trade union. It determines entry fees for apprentices, selects masters, and bands together to pay for things such as festivals, member's funerals, weddings of members, etc. It is a sort of group-fund for retirement and large expenses as well as an advertising organization for its members in good standing. Guilds represent a good source of collective wealth, and like unions are also ways for craftsmen to develop and exercise political clout.
True tradesguilds came into being in the High Middle Ages. Before that, they were mostly organizations of tradesmen of a single type but without the rigid controls one might see in the 14th century guilds of Italy, for example. As I am an early medieval historian, this is the way I run my guilds—a sort of amalgamation of the Roman craft-guilds and early medieval ones which provide money for holy days.
How can a Guild exploit capital?
In the traditional Roman and early mediveal sense: it cannot. Guilds do not have the power to "sponsor" new business. They can help a fellow tradesman recover after a fire (though he would probably be able to rebuild his house himself; building your house was generally your own affair) or an apprentice after the death of a master but they really have no role in financing long distance operations. For that we'll need...
Costers — What are they?
The definition of Coster that I'm using is one that I've extrapolated from the Forgotten Realms, which has many of these organizations. The word in and of itself in English means a fruit vendor in a market, but Greenwood uses it to mean the following:
A loose organization of merchants who pitch together for common good, often alleviating the burden of dangerous mercantile ventures by common interest.
Costers are wholly fictional, but operate in some ways between a Joint-Stock Company and a Guild. They serve as a means for merchants who are already wealthy to band together, invest capital in a venture, and then reap the rewards without levying undue burdens on any single merchant if a ship or the whole fleet should sink. Same goes for caravans.
What makes this different from a Joint-Stock Company is that there are many ways the Coster might operate: it may be a corporate entity in which individual merchants pay dues and therefore receive cheaper use of dock facilities where the Coster operates. It may work as a trade union does to enact political reform in favor of the merchants. It may provide a central treasury from which guards are paid, etc.
The Joint-Stock Company
The most advanced of these options (in time) and the one I never use is the Joint-Stock Company. These are corporations formed of a number of shareholds, each of whom "own" a share of the company and invest a substantial portion of their personal fortune into its success. These were, notoriously, used during the Renaissance to facilitate the exploration of the so-called New World. While I personally find them too legalistic and too late, I'm sure you can get a lot of mileage out of Joint-Stock Companies—hell, you could have a Joint-Stock Adventuring Company who work for chartered shareholders!