Well... if your city is as large as Oligarch, you can summon 40k CS worth of peasants. That is about 8000 peasants. The sheer number alone and destroy most player fielded army. And it is literally free. You can use it as long as there are no peasant and your ruler has enough prestige. You can just invest afterward to repair the damage.
How does this work, anyways? Just in the capital?
