primary source of money are cities specialized as towns. they get higher base income with each level up and unlock the market and tax office buildings which further increase income. and they have access to a line of buildings (well>inn>festival) that increases money income by a percentage.
so towns are your safest bet. secondary sources of money are the "propaganda" city enchantment of cities with several essences (+1 g per essence), gold mines that can sometimes be found as bonus resources (similar to shards, iron mines, crystal crags etc.) and of course also the loot you get from killing monsters and clearing lairs (this includes selling unneeded items at the shop of a city).
and you can make quite a bit of money though diplomacy. each trade agreement you sign gives a 10% boost to your income, so try to make some friends and sign those treaties (preferably AI nations of your side - kingdoms if you're a kingdom, empires if you're an empire - the "enemy" side tends to get unfriendly/hostile later on)
and you can also sell unneeded resources (excess iron for example) or tech knowledge to AI factions for gildar.
i think that's about it.
