During the Early Middle Ages, the Colosseum was used as the site of a church and cemetery. Later on, the spaces in the structure were rented out as shops and living quarters. It was also clearly used for other purposes, though the records are not complete about this, and parts of it were removed to be used elsewhere.
the reason there are pieces missing is because people have been taken the seats because of the stuff the seats have been made of. and there was fires and earthquakes so it destroyed two thirds of the outside
The Colosseum was originally used for entertainment purposes. During the middle ages it was used as a source for building materials and today it is one of Rome's biggest tourist attractions and it is actually the symbol of the city of Rome.
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
knights
to entertain
YES
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.
It was used in the middle ages by monks!