Towns began to develop and increase in trade, which made the towns wealthy. This lead to the decline of feudalism because these towns resented the noble's rule and became independent.
King John overtaxed his barons and took away their rights. He also got in fights with the church.
So his nobles forced him to have a meeting and sign the Magna Carta. This led to the decline of feudalism because the people were given power and the king wasn't above the law.
The Black Death killed 24 million people living in Europe. This lead to the decline of feudalism because it changed Europe's economic and social structure very quickly.
The Hundred Years' War lasted for over 100 years, at first, the English were winning, until Joan of Arc lead France to victory after God told her to lead France to victory.
Joan was burned at a stake after she was captured. This lead to the decline of feudalism because the English lost the war. And this led to kings not needing knights for armies, castles becoming useless, peasants feeling more loyal to local lords, and common people having power and influence.