This time period can be very confusing, I understand it's a long timespan involving a lot of diverse regions and cultures, but there's still normally some common ideas presented when you try to seek information about it. At times it seems the fief is a very primitive structure with limited activities. In other moments you are presented with information that makes you question that.
Usually you find that serfs had to pay a percentage of their produce to their lord and work on his property occasionally. There is also information you find around on the internet about several taxes and fines the peasantry occasionally had to pay. Sometimes the information makes it seem like each fief has a considerably self-sustaining economy that functioned on bartering, so were the fines paid with further agricultural produce or with actual coin? If so how did the peasantry obtain coin if they were bound to the land and were limited to work in their land? The examples you find on the internet usually come down to agriculture, but obviously there were other enterprises, as wood, leather, milk, meat, iron and other materials had to come from somewhere. Obviously someone had to make clothing, shoes and the weapons the nobility used in battles and other more elaborate crafts as well. Were the serfs responsible for that as well or was it other ranks of peasantry outside the fief? Did they have to pay a share of what they produced as well? If so did they sell the surplus for actual coin or just exchanged for other goods?
Let’s look at the economy of the region of the Uzege, in Gard in the South of France in about the 13th century.
This region had about 30 villages, each supporting a castle and a knight (Seigneur), with an average population of about 400 people. At the center of the region was the small city of Uzes. The city has 8,000 people today. Perhaps 5,000 in Medieval times? (6,150 in 1793). So, the total population of the Uzege was about 17,000.
The country people were mostly farmers. The agriculture of the area, then as now, was vines, olives, fruit, vegetables, grain, sheep and goats. The villages produced both primary products, but also secondary products, such as wine, olive oil, cheese, charcoal, spun yarn, woven cloth.
The village economy might have supported a baker and a blacksmith, a few carpenters and masons, perhaps a potter (making Roman tiles for roofs, as well as pots), if there was good clay nearby, and a priest.
Not all villages had a mill, but many did. The village might also have a wine press and olive press.
If there was a mill, the Seigneur generally owned it, and a Miller ran it. The miller took a percentage of the flour as his fee, and paid a share to the owner as ‘rent’. The Seigneur might also own the wine and olive press, and charge a portion of the production for the use. (The villagers could press their own grapes by foot, but it was advantageous to use a basket press and pay, because the basket press will produce 15-20% more juice than foot pressing.)
https://en.wikipedia.org/wiki/History_of_the_wine_press
The Seigneur, probably received a labor levy from the villagers (40 days was a traditional Medieval levy – but no record if this was the levy in the Uzege), and some portion of the villagers produce (the church traditionally got some portion as well).
In some places in Medieval Europe, feudal landowners had the right to run a court (and collect a percentage of court decreed fines). Don’t know if Seigneurs in the Uzege had village courts. There was a court in the city of Uzes.
There would have been a market in the city of Uzes, and villagers would have come in to the market to buy and sell and trade once a week. The market was probably run from booths or stalls or carts or wagons in the Place aux Herbes, much like it still is.
We know that the bishops of Uzes had the right to coin money, so there was probably some form of money economy, not just feudal barter, even as early as the 12th century.
The city was surrounded by a defensive wall (now gone and replaced by a road). Inside the city were three great defensive towers (still there), The Tour Bermonde (the keep of the Seigneurs de Uzes - first recorded in 1080 AD, who later became Lords, titled "Viscounts", "Counts" and then "Dukes"), The Tour du Roi (built by the town guilds as a counter power to the Lords and the Bishop - later taken over by the Kings of France), and the Tour de l'Eveche (The tower of the bishop's palace).
There was a bishop of Uzes since at least 442 AD. There was a cathedral there in the 12th century. There are also several other churches.
In the center of town is the "Place aux Herbes" which is where the market was (and still is) held.
Buildings in the villages and the city were of stone, with Roman tiled roofs.
The stone, lumber, and clay to build houses, barns, castles and churches was all to be found on the lands of most villages.
Domestic buildings in the villages were generally two stories. In the city, most were typically three or four stories high. They frequently had a vaulted ground floor, used for workshops or storage or shops, with living quarters on the two or three upper floors. There were a mixture of apartment buildings, single family homes, and the city mansions of rich merchants or feudal knights from the surrounding countryside.
The main industry in the city, probably since the 12th century, was the manufacture of cloth. (People in the villages might weave rough homespun cloth, but finer cloth was produced by dedicated weavers in Uzes). The city of Uzes was also a center of government, church, law courts and commerce for the surrounding area. The bishops had the right to mint coins in the 12th century, so there was a mint.
On the outskirts of the town were several large monasteries. The town was surrounded by the hunting preserve of the Lords of Uzes, and by farmlands and small villages, each with its own small castle.
Textiles was the largest industry, so many of the vaulted ground floors would have housed hand looms, or warehoused yarn or cloth. There were undoubtedly other workshops in the city as well. (Carpenters, Masons, Blacksmiths, Coopers, Dyers, Fullers, Tailors, Tanners, Leatherworkers, Cobblers, Cabinetmakers, etc.) There were also wine and produce merchants (Wine, flour, olive oil, were produced in the country villages - or the grain mills on suitable rivers. Lumber, quarried stone and clay tiles would also have been produced in the countryside but consumed in the city. One of the local villages, San Quentin de la Poterie, specialized in making pottery ware, which also would have been sold in Uzes), butchers and bakers.
These sorts of establishments would be run by a master craftsman, perhaps with some journeymen and apprentices, plus, of course, a large contingent of unskilled laborers, who moved things and carried things and worked for the merchants and skilled craftsmen.
As an administrative center, there would have been clerks (who could read and write) and some lawyers. The Seigneurs of Uzes would have had a small group of men at arms. There would be a number of monks and nuns in the monasteries on the outskirts of the city. Also, the richer inhabitants would have supported a fair number of domestic servants. The very rich, such as the Lords of Uzes, or the Bishop, would have had some specialist retainers, huntsmen, falconers, grooms, chefs, perhaps some landless knights who served the lord.
There were metal workers, both working in the mint, but also casting or forging metal implements and weapons.
In short, in the 1200s in the Uzege, one would find an economy based on the countryside producing primary products and some secondary products, and the city producing finer secondary products and services.
/u/mormengil has already given a very detailed example of a fairly average region for the time, so I'll just add that the "taxing" itself came in many different forms and varied greatly according to time and location, but in general, there was a tendency to gradually replace payment in kind with payment in money, by the peasants to the lord. This fueled smaller regional markets, which in turn contributed to greater monetisation in general. In addition to this, the corvée, which is unpaid labour, saw a steeper decline than payment in kind (with which it coexisted for a long time), since most landlords realised that letting the farmers work their own lands full time was more efficient, and it eventually disappeared in many places.
In some cases, this decline of payment in kind and unpaid labour was not directly replaced by monetary taxation on the produce, but by indirect taxing, of which there are hosts of types (on inheritance is one of the more famous ones, but they were countless and coexisted in many different combinations). These transformations were often source of conflict, and often led to court disputes. I remember a specific one where the villagers of Limonta, in northern Italy, went to the courts to dispute the claim made by their new lords (an abbey) that they had already offered unpaid labour since the days they were under Lothar.