DNA to the rescue!
After you finish the main story mode, whenever you enter the game again, each kingdom will have a "bank" in it. You can send any or all of your warriors in a kingdom into the bank (but doing so ends their turn). When you send them in, you can pick a location in your kingdom to upgrade, then spend either 500, 1000, or 2000 to upgrade it; then, the higher their combined Wisdom score(s) are, the more it will be upgraded.
The max level for any facility is 3, and if you send in warriors with a combined Wisdom score of about 350, you can upgrade by 1 full level by spending 2000 (and then do it once more to get it to level 3). The banks have no level.
I recommend maxing out a gold mine first, so then you'll have more money to spend on upgrading places elsewhere (plus a bigger source of income).