Castles over a moat.
No-one knows. Forms of the drawbridge were used, albeit rarely, as far back as Ancient Egypt, 3000 BC. They did not become common until the Middle Ages in Europe. Leonardo da Vinci designed some - he is not the inventor though.
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 Aztecs used a combination of wood, stone, and ropes to build drawbridges. They would create a framework using wooden beams and stone supports, then use ropes to attach the bridge to a mechanism for raising and lowering it. These drawbridges were commonly used in their cities to span canals and waterways.
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!