I'm just barely beginning to flesh out the structures of power in my story's world

The Arekean Empire wasn't always an empire (and if you ask some people it still isn't). It began with the magistrate of a single autonomous city state leveraging the vast quantities of debt owed to her to get the neighboring nobles to bend the knee and swear fealty to her instead of the current monarch.

Over time the new province grew and founded colonies beyond its borders. Eventually those colonies grew into proper city states, equal to or even superseding the capitol city. A brief civil war deposed the original "Supreme Magistrate" and saw the rise of a more equal federation where the republic was divided into provinces, each ruled by a magistrate. Though the house that led the charge in the war was given the title of "First Magistrate," the office was much less powerful.

The republic slowly morphed into an empire over the next 200-ish years of repeated phases of conquest and peace, and completed the transition during the last time the empire faced full scale invasion. Empress K'rea the emergency as an excuse to crown herself to cut through red tape and more effectively lead them through the crisis. The next 500 years of imperial politics can be boiled down to a three way tug-o'-war between the crown, the magistratta, and the nobility (though the nobility usually lose in both directions).