A player in my Sertorius campaign recently pointed out that some aspects of the game sneak up on you. Reading the rulebook it is not always immediately clear what a chaotic influence Sertori are on the world. This is discussed in the GM chapter, in key places of the Gazetteer and in the history section but it is somewhat buried. So a lot of players are surprised when they start their first campaign by how powerful the party is and how easily their characters can shake up the politics of Gamandria.
In Sertorius, characters are strong out of the gate. They begin with abilities that would place them at mid-level in many other systems. This means they become important quickly, they rise through the ranks quickly, they disrupt the balance of the world quickly. It is not uncommon for a party of Sertori to begin seizing power for themselves very early in a campaign.
This does depend on a number of things. There are considerations the GM needs to weigh when dealing with parties on the rise. Not every campaign will handle this in the same way.
The party's spells are crucial here. Sertori begin with four spells, so the spells they choose matter. In a party of four or five characters, how their spells compliment one another is significant. Most parties, even if they do not possess obvious combat spells, can use their magic cleverly enough to work situations to their advantage. But there are some combinations that are better suited for taking crowns than others.
Another important factor is location. The more civilized places of the world usually have institutions in place that keep Sertori in check. They simply have more experience successfully managing Sertori and have built on that (and these institutions are typically made up of Sertori themselves). For example The Caelum Republic has an order called the Caelcori. These are all Sertori and they function as a kind of secret police who enforce an ancient law preventing spell casters from holding certain offices. There is also the Fellowship of Promestus in Ronia, and the Phra Jao in Khata (a kind of governing body of Sertori). Frequently the Sertori themselves are in control (as one might expect). The ruler of the Mandaru Empire is a Sertori for instance. In these cases, there is a certain level of stability in the relationship between Sertori and non-Sertori. Away from places like this, things get considerably more chaotic. Even where there is a Sertori in control, it only takes a more powerful Sertori or group of Sertori to buckle the established hierarchy.
The presence of other Sertori is also something GMs need to consider. Many cities and kingdoms are ruled by Sertori. Where Sertori do not rule directly, there are Sertori behind the throne, supporting it and protecting it. Taking power is not usually as simple as killing a mortal king and his guards, it can be quite a dangerous undertaking.
Non-Sertori themselves are still able to kill Sertori. While Sertori can cast spells and are physically more powerful than normal people, they are not invulnerable and they can still die. A hundred soldiers firing arrows at a Sertori will kill him unless he has a particularly appropriate spell to the situation. Six wounds. A Sertori can sustain six wounds before dying. Non-Sertori who plan ahead can easily manage that.
The total population and distribution of Sertori is another key consideration, and this varies from one campaign to the next. The number of Sertori in Gamandria was left deliberately vague in the rulebook. We gave a possible range of 200-3,000 Sertori existing in the setting. Ultimately the exact number is up to the GM. If there are only 200, that means a lot of places won't even have Sertori (because the Caelcori and the Fellowship of Promestus alone take up about half that number). In a campaign with very few Sertori, there are more opportunities for PCs to wreak havoc but also a greater assumption of stability. In a campaign with more, there may be fewer opportunities and more impediments, but likely a greater degree of chaos (though perhaps not, this was a subject we debated a lot and the conclusion was not always obvious or agreed upon by all three designers).
I encourage GMs curious about campaign power levels in Sertorius to read CHAPTER THIRTEEN: GAMEMASTERING GAMANDRIA and to take a very close look at CHAPTER NINE: PEOPLES AND PLACES. In particular I suggest reading the following entries to get a sense of the impact Sertori have on the setting (and their limitations): Trade Routes (235), Atroxis (257), The Caelum Republic (264), Donyra (274), Khata (281), Mandaru Empire (283), The Marite Kingdoms (285), Matruk (286), Midbar Valley (288), Ogre Gate (290), Phra Goa (291), Qam'ua (294), Ranu and the Ranu People (295), The Ronian Empire (298), Tajem (314), The Taksiri Pirates (315), Talyr (315), Traya (318), Tungat Oasis (318), and Ubara (319). Another section of the book worth examining closely is CHAPTER EIGHT: RELIGIONS, ORGANIZATIONS AND TEXTS. It includes many entries relevant to Sertori including The Children of Nong Sai (227), The Cult of the Emerald Serpent (227), The Cult of Karima (227), The Cult of Kwam Jao (227), The Cult of Sukat, The Caelcori (230), and The Fellowship of Promestus (230). CHAPTER TEN: HISTORY AND LEGEND also illustrates how Sertori have shaped Gamandria in the past.
In the end this is your setting to use and tweak as you wish. Nothing beats thinking about the nature of power and deciding things for yourself.