Break away cities
I was pondering the possibility of breakaway cities--- cities that are part of a channeler's empire that aren't particularly loyal, and will switch sides or declare independance under a variety of circumstances (too much oppression, their channeler is losing a war, not enough money is spent in the city despite high taxes, the city having belonged to another channel or was neutral in the past, etc.) A garrison of soldiers of the channeler's soldiers would obviously discourage such a breakaway.