Everything Starts with Grain
You can tell a lot about the way a culture will develop when you figure out what they eat. Since most fantasy-era RPG’s are set in a quasi-medieval setting with established town centers, the important food factor is what sort of grain the people are eating. Agrarianism is a prerequisite for stable settlements, and once the locals are harvesting grain, you have the luxury to develop other ideas, and the necessity of a system of supervision and food distribution that doesn’t exist in a hunter/gatherer setting where everyone’s primary duty is to get enough food to feed themselves every day. In a very real sense, the actual base economic unit of the preindustrial society is not the coin, but the bushel of grain.
The first thing influenced by food is how many people can be supported given a certain amount of arable land, and how much of the population must be dedicated to farming. A primary crop of wheat can support a certain population per acre, barley a different number, oats different again, etc. Because game designers tend to be unimaginative, they tend to use wheat as the primary crop if they’ve even bothered to think that hard about it. A general, historically accurate figure is that each wheat farmer produces enough food for himself, his 3 non-farming dependents (who don’t get as much food as the farmer), plus 10% surplus, taking into account the amount of grain you need in reserve for replanting. This means that for every non-farmer (adventurer, politician, soldier, etc.) in town, there must be 10 farmers raising wheat. This figure reflects the best technology available in a preindustrial society prior to the late 18th century, where advances in erosion control and fertilizers increased yields. The sort of technology we are talking about here is the kind you would expect to see in a fantasy quasi-medieval society: heavy plows and the wooden horse collar. Wheat farming has the considerable advantage of encouraging draft animals and horsemanship with increased hay and feed production, which in turn leads to cavalry. Each farmer generally works about 7 acres of wheat, and assuming your society is using a three-field system, each farmer would require about 10 acres, including the land that is left fallow that year (generally used for grazing while the land replenishes itself). Using a rough conversion of acreage to square miles (640 to 1), each square mile of wheat will thus support 64 farmers and their families plus 6.4 non-farmers. A typical farming village in old England of about 180 people thus requires 3 square miles of arable farmland, which is in keeping with the fact that such villages typically existed about 2-3 miles apart.
However, if you are trying to design a civilization that has relatively tight borders and supports a gigantic number of people, you have to either say they trade for their food from less populated farm regions (raising the cost of living for everyone in the city) or say there is a very high-yield crop/farming method that allows for large population support on smaller acreages. Corn is an extremely high-yield per acre crop, and can be used as your primary food source, although you have to get around the fact that corn is very susceptible to blights and such in an early farm culture. The middle american cultures subsisted almost entirely on corn, and its efficiency allowed them a spectacular amount of leisure time to develop technology, but every few years they had to contend with massive starvation because a crop went poorly. Rice can also support a tremendous number of excess people above the number of workers required, but rice is a bizarre crop that allows for a virtually unlimited number of workers in a small area, each of whom produces just slightly more than what he needs to survive himself, working throughout the year in several harvesting cycles. This makes it attractive to small landmass communities like those of feudal Japan, but creates a whole new set of social implications. This will be discussed later.
Rice does not encourage the domestication of horses, hence extensive, specialized cavalry is not a natural outgrowth of rice communities. (This can be convenient for a game designer who doesn’t want to be bothered with horses.) Horses were used in a few non-wheat based cultures, notably the Mongols and the Japanese, but they fed on available scrub, and they never reached the level of universal application or breeding as a wheat culture’s horses. For instance, neither the Mongols nor the Japanese bred specialized draft horses as did the Europeans, thus they never got heavy warhorses, thus no close formation lance-using heavy cavalry.
It’s conceivable that corn could encourage horse domestication, but there is no historical reference for this, since horses were not available in the new world until introduced by the Europeans. However, the use of horses is quickly learned when they become available. The Sioux people almost immediately became a culture based entirely on horses when they were introduced.
Supplementary food sources also have an effect on your maximum population per acre of arable land. Generally, the typical inland European diet of the middle ages was very light on meat, at least for the peasantry. Upper classes would demand more meat, but you don’t have to get too deeply into detail here. The thing to keep in mind is that herding animals that require grain to feed are roughly 1/10 as efficient as the raw crops in terms of pastureland. Therefore, if you want to have the typical fantasy idea of a roasted haunch in every tavern, you need to allocate even more land to pastures. However, to make up for this there are some land-efficient methods of getting meat into the food chain. Pigs are typically left to run in contained forest areas to forage for themselves, effectively harvesting nuts and roots (and garbage in the form of slops) and converting it to ham and bacon. Game can be taken, of course, but if you go too heavily on the idea of game you deplete the forest and run out of game in subsequent years, thus hunting cannot be relied on as a major source of nutrition. Cattle and the like can graze on the town commons, saving a little bit of pasture area.
Fishing can be incredibly efficient in areas that can support it, taking up no land at all and returning large amounts of protein, especially if your civilization has advanced fishing technologies in the form of nets, boats, and possibly even fish traps. A fishing village can generate up to a 150% surplus, and fishing industries helped to fuel the prosperity of the early Normans, and the rise of a better-fed and richer middle class. North american tribes along the Delaware river were able to harvest as much as 20 million tons of fish annualy, raising their standard of living considerably. However, people tend to get sick of fish very quickly, and there should be an alternate food source. In modern-day African fishing villages, dogs and cats are cosidered edible and desirable as food, and can be traded for goods and services.
If all else fails, you can use black box devices like "magical weather control and soil refreshment" which allows for more than one crop of wheat to be harvested per year, and negating the necessity of a two or three field system, i.e. no farmland is ever fallow. However, an unrealistically superior food source which is easy to harvest, i.e. massive amounts of fruit on every tree, tends to lead to a sedentary and primitive society due to a lack of need for innovation and industry. Jungle communities that subsist heavily on readily-available fruit tend to stay in the stone age while the rest of the world is forced by necessity to move forward.
Once you have your food sources determined and a supportable population figure, you can tell a lot about local political systems from what your crops consist of. In the case of wheat, barley, and other annual grain crops, this contributes to the feudal system of "Lords of the Land." When you have food and land to grow it on, someone is going to inevitably try to take it away from you. This results in the creation of a warrior class, dedicated primarily to holding onto the farms, and maybe taking the next guy’s fields as well. These warriors are excess people, and don’t produce food themselves, taking it instead from the farmers. Now your army becomes somewhat organized under some form of leadership, and because they have the power to keep the peasants breathing, they naturally assume a leadership role, sometimes going so far as to bar the peasants from owning proper war gear out of a sense of job security. The farmers keeps the warrior class from starving, the warrior class protects them from invaders and wild predators. Historically speaking, wheat farmers and the like stayed in their appointed social station, only rebelling against the lord when they were taxed so heavily that they began starving to death. The lord of the land, for his part, typically taxed the peasants as heavily as he could possibly get away with, but woe was the lord who starved his farmers to death. (Example: Wat Tyler’s rebellion, 1381.)
Rice and the methods required to grow it bring about a different social system. Because rice is a relatively low-margin food source (i.e. very little is produced in excess of what the peasant requires to live), you need a phenomenal number of people working relatively small rice fields if you want to support a warrior class to keep your people safe. This requires a very advanced and strict management system in which everyone must obey the taskmaster, or the whole village will starve. This is likely the origin of the strict disciplinary tradition of the early Japanese people, which continues to influence the culture even today.
Taking our example of fantasy supercorn, we have here a crop which is exceptionally high-yield, and relatively few farmers can support a large number of excess people (say, the population is merely 60% peasantry, as opposed to a more realistic 90%). The social implications for a supercorn farmer are significant. On the one hand, this production of excess crops can mean the farmer has more freedom of choice and status, especially if supercorn is hard to cultivate properly. If this is the case, the farmer who can get maximum yields out of his supercorn has been elevated from unskilled laborer to desirable artisan. The growers of supercorn may even have enough clout to form a guild, but this is unlikely; food is such a basic requirement of society that any attempt to "strike" or price-gouge by supercorn farmers would probably lead to the warrior class beating them down. If hedge-wizards are required to get a faster crop rotation, then these specialists may have political power and influence, although they would almost certainly be civil servants and not player characters. However, the political intricacies of supercorn farming NPC peasants are not really important to the player usually: it’s just something to keep in mind when creating a believable social system.