Wealth means power, power means empire building. Therefore, a steady income is crucial in every campaign game. In his thread, Lord Ahm has initiated a discussion about economic aspects of RTW. The first paragraph in his post concerns upkeep and garrisons:
The army upkeep is divided on all your cities. For example: You have a huge army in Arretium, that army needs money to maintain itself. The army upkeep in that army will not be paid by Arretuim, just because the army's location is in Arretuim. The upkeep expedenture will be divided to all your settlements, and the one with most citizens will pay most money.
Thats why your bigger cities are loosning money, the city have so many citizens that they pay a large part of the army upkeep.
The thread is definitely a worthwhile read. If you have something you would like to add, or any questions, visit the thread!