During the Middle Ages, nearly all labor was provided by serfs. Most medieval countries did not permit slaves. Serfs were mostly farmers, but they worked in transportation, manufacture of various products, helping masons and carpenters on buildings, doing forestry, and really any work that was simple labor.
We all know this is homework or schooling.
Castles in the Middle Ages were used to keep out enemies.
See the question: What are some weapons used in the middle ages?
In the Middle Ages
people in the middle ages used letters or talking to face to face
because they used wood smoke as a deorderant in the middle ages
The catapault was used mainly in war. The catapault was made in the middle ages, a kind of cousin of a cannon.
knights
YES
to entertain
It was used in the middle ages by monks!
Yes, polyphony was used during the Middle Ages. We do not have any real idea of when polyphony was introduced, but the earliest texts describing it date to the Early Middle Ages. By the end of the Middle Ages, it was highly developed. There is a link below to an article on polyphony, beginning at the section on its origins.