What was the Praetorian Guard?

a praetorian guard guards the king in the middle ages but most of them usually betrayed their king and killed them because they had to do everything

The Praetorian Guards guarded the Emperor's of Rome. They were elite warriors and were also very well paid. Although they initially served their purpose as guards of the Emperor and Rome, they rapidly gained extreme power and became not only the Guards of the Emperor but also a source of support - Many Emperors were established because they gained control of the Guard, or deposed because they were not being paid enough.