The choice of background and culture in your medieval fantasy is a hugely influential one on your later writing. Obviously, if you choose to model your world directly from the real one, there are going to be a number of clear differences in advancements in technology, spiritual belief, music, politics and general behavior between countries.

That isn't to say you need to keep worlds separate. The 14th century writer Chaucer famously wrote The Canterbury Tales from the cultural perspective of medieval England, however the Knight's tale is set in ancient Athens. Despite being set in another country the story revolves around English hierarchy, courtly love and the typical aspects of Knighthood like chivalry and jousting for fighting. However it does touch upon Greek schools of thought such as Stoicism and ideas from philosophers like Plato. There is much artistic licence to be had.

Now, I've had a specific request to take a look at 'medieval' Korea, so let's get started, shall we?

Before you research, it's probably a good idea to pick a dynasty. The Goryeo Dynasty (고려) is from 936 to 1388, while the Joseon Dynasty (조선) is from 1392 right up until 1896. Depending on which you pick, you will have to make different cultural references in your writing. If you want to make your life easy while researching, pick a very specific date to set your story in. Here's a short timeline of important happenings to give you an idea of what it was like when. (source)

Goryeo Dynasty

956: Emperor Gwangjong forces major land and slavery reforms, and in 958 implements civil service examinations.

1033: Goryeo builds the Cheonri Jangseong, a massive wall running along the northern border.

1170: Military coup.

1231: The Mongol invasions of Korea begin.

1234: Choi Yun-ui's Sangjeong Gogeum Yemun is published, world's first metal-block printed text.

1251: Goryeo completes the Tripitaka Koreana, the most comprehensive and oldest intact version of the Buddhist canon in Chinese script.

1270: Goryeo signs a peace treaty with the Mongols, beginning an 80-year period of suzerainty. The Sambyeolcho Rebellion lasts for three more years.

Joseon Dynasty

1396: Capital moved to Hanyang. (modern day Seoul)

1402: Paper currency initiated.

1409: First Hwacha (multiple rocket launcher) created.

1446: The Hangul alphabet, created 3 years earlier, is promulgated by King Sejong the Great. REMEMBER: They wrote using Chinese before this, using Chinese so much that the merchants used Chinese numbers (now a warped Chinese known as Sino-Korean) presumably to make transactions easier. After King Sejong implemented this, the literacy rate went up to pretty much 99% and has stayed that way ever since due to the simple and phonetic nature of the single alphabet, easy to learn quickly unlike traditional Chinese pictoral writing which one you could spend decades learning and not know every symbol. This is a pinnacle point in Korean history.

1592: The Japanese invasion of Korea begins under the command of Toyotomi Hideyoshi. Admiral Yi Sun-Sin employs the Turtle ship to repel Japanese naval forces.

1627: The first Manchu invasion.

1636: The second Manchu invasion.

1653: Dutch ship, with Captain Hendrick Hamel, wrecked on Jeju Island.

1791: Persecution of Catholicism begins.

Fashion

The traditional Korean clothing was the Hanbok. To get a more realistic idea of the colours they would have been, take a look at some period paintings of the aristocracy. However, it's not unreasonable to say they would have been some quite bold colours for the higher aristocracy. The word Hanbok refers to "traditional" clothing nowadays, but back then it just meant "clothes". (Further reading)

Politics

During the Goryeo period laws were codified, and a civil service system was introduced. Buddhism flourished, and spread throughout the peninsula.

At the beginning of the Joseon Dynasty, the country's official religion was listed as neo-confucianism (read more). Royalty alongside a crown court ruled a stable bureaucratic state. Confucian scholars held a high position in society at this time, whereas before it was the skilled in martial and military skills who were held in highest esteem before the intellectual.

Culture and Society (exerpts from wikipedia; Joseon Dynasty)

Many documents were written about history, geography, medicine, and Confucian principles. The arts flourished in painting, calligraphy, music, dance, and ceramics. (video)

During Joseon, a social hierarchy system existed that greatly affected Korea's social development. The king and the royal family were atop the hereditary system, with the next tier being a class of civil or military officials and land owners known as yangban, who worked for the government and lived off the efforts of tenant farmers and slaves.

A middle class, jungin, were technical specialists such as scribes, medical officers, technicians in science-related fields, artists and musicians. Commoners, i.e. peasants, constituted the largest class in Joseon. They had obligations to pay taxes, provide labor, and serve in the military. By paying land taxes to the state, they were allowed to cultivate land and farm. The lowest class included tenant farmers, slaves, entertainers, craftsmen, es, laborers, shamans, vagabonds, outcasts, and criminals. Although slave status was hereditary, they could be sold or freed at officially set prices, and the mistreatment of slaves was forbidden.

Folklore

This is the coolest folklore PDF you've ever seen in your life. Trust me. (click)

Wikipedia is a rather eyewatering goldmine of Korean mythology. (clicky)

If I tried to cover this topic myself, we'd be here all year. Do some wider reading on this one; it's thoroughly entertaining.

Language

When writing dialogue, it is important to fully understand how status and 'closeness' of the speakers would influence their mode of address. In Korea, there is a long tradition of honorific speech, and methods of politeness which involve maintaining 'face'. Face here does not refer to the features upon your noggin, but one's dignity or prestige. (read more)

Architecture

Korean buildings (hanok- house) are iconic in an of themselves. The lower classes would often have much smaller, thatch-roof houses, but similar wooden design walls. Each house would have a designated area for large terracotta pots where food like kimchi was stored and fermented. People typically slept on the floor and sat on mats at low tables. The houses were heated with ondol, underfloor heating unique to Korea. They also have a wide front porch to keep the interior cool in hot summers. Social status is important to the grandure of the house. Upper classes typically had tiled rooves. Some also believed in different Gods relating to each house of enrollment.

(images)

Fantasy in Action

There are many books and movies depicting medieval Korea, whether they be fantasy or not. It's good fun to see how others have depicted times past. Remember modern media pieces are also constructions and can't possibly be 100% historically accurate. Some movies include The Divine Weapon, Woochi the Demon Slayer, and A Frozen Flower.

Wider Issues

Interesting cultural debate is to be found within Korea's past. ism, the relationship with China, and the hereditary class barriers are big ones, and relatively easy to research. Did you know that not long ago, was a crime against a woman's chastity rather than the woman herself?

If there's a tradition or event you want to depict accurately like a birth, a wedding, a funeral, new year, chuseok... use google. Or, you know, a book of some sort; they tend to go into much further depth. I implore you to research customs and way of life before writing it. Everything accurate or detailed you include adds to the depth and substance to your albeit fictional world. Understanding how your world works rather than just imagining it will make it go from 2D to 3D.