What was medieval warfare like? (feat. Overly Sarcastic Productions)

What were the distinctive elements of warfare and battle in the medieval period? Why were castles built: https://www.youtube.com/watch?v=U6nq6nFaCWM Overly Sarcastic Productions video on Classical Warfare: https://www.youtube.com/watch?v=UMpCDwOGZzU Shadiversity T Shirts: https://teespring.com/stores/shadiversity