[Taqwus]

Black Cat --
The economic model's not that complicated compared to, say, Civ. Basically, there's gold and resources.
Gold: shared in a nationwide treasury. Generated by taxpayers, certain magical sites (including mines; you can't build mines, but can get them randomly), alchemy (turn fire, earth gems into gold), random events. Spent on units (generally reflects rarity/training), unit maintenance, building construction (temples, labs, castles), province defense, repairing damage from random events.
Resources: local. Used in province and adjacent provinces if they have castles. Does not accumulate -- use 'em or lose 'em. Required for training most units; amount reflects equipment e.g. knights require a lot, barely-armed militia very little.
Castles: Improves resource utilization (locally), sucks in resources from neighboring provinces, boosts tax income. Boosts related to administration value. Also permits production of national troops, with exceptions (underwater/land difference).
Taxes: Rate defaults to 100%. High taxes result in less taxpaying population due to people fleeing, starving, and being killed by your patrols after they revolt. Low taxation levels may ease unrest.
Order, production scales affect taxes, resources respectively as well. Growth may, don't recall; it has a long-term effect on no. of taxpayers as well. Heat/Cold affect taxes negatively if your people don't like the local conditions.

There's no investing in, say, trade guilds or loans or banking centers or what-not. It's not a civil-development game.