I like this system, but why must one be a magistrate before prefect? I thought prefect is much low-ranked?
Not the County Magistrate comparing to Perfect in China.
County Magistrate (Xianling 县令) - The lowest level government official in China -- comparable to the mayor of a city, but it was more like the official person who was responsible for the Civil affairs in a county or a city -- mayor + judge + prosecutor + police chief (sort of). He was also the the 7th grade (the lowest grade) official in the Chinese government. Anyone below that rank was NOT considered being a government official truly, just a government employee (like the Shi Yei, legal secretary, or Bu Kuai, the policemen, etc.)
Prefect (Taishou 太守) - Usually higher ranking in Chinese government. A lot of time, he had both Civil and Military responsibilities as well as having larger region to control than a mere County magistrate!
I know this because my father was one of the lowly County Magistrate in early Republic era in his 20's and 30's, and he had explained the particular responsibilities of the County Magistrate to me (and its horrors of having to worry about every single thing of the Civil affairs while trying NOT to offend the local rich and powerful people because everyone of them are/were probably more powerful than a mere County Magistrate!!!

) If he was a Perfect, the powerful people had to beware of him because he would have the control of the military and would be considered a local warlord or military governor or something like that -- Then, NO ONE would dare to offend him!!!