How did Commodus keep power?

Answer 1

By the same means that the First Citizens had for a couple of hundred years - the system established by Augustus was: 1. Taking the powers of a Tribune of the Plebs which gave the right of veto, the right to introduce legislation, and personal immunuty. 2. Taking the powers of a magistrate outside the city of Rome, which gave personal immunity and the right to direct affairs throughout the empire. 3. Allocating to himself the frontier provinces, which was where the legions were stationed, so he had control of the army.