Gunpowder and the longbow changed warfare because were able to pierce through knights armor easily killing them. This made the knights expensive suits of armor worthless.
Decline in Feudalism
The Bubonic was one of the main causes for the fall in feudalism. This happened because it killed many serfs, peasants and other workers causing work to be very expensive. It also killed many lords having the king to become very powerful.
Black Death Diary Entries
Barter System
Powerful lords brought peace and stability around the year 1000 that helped the market grow because they help them stay safe like creating safe trade routes helping people expand and tell other info.
Some flaws in the Barter system was if you needed something but the person who had it didn't want your thing you would be out of luck. In a worse case nobody could want what you had. People could also be paying not enough for the item and not making a fair trade causing you to lose money. Finally there was little capital gain to the barter system.