In each kingdom, there's a Bank. You can send one, any, or all of your warriors in a kingdom into a bank - the more their combined Wisdom scores are, the greater of an effect it'll have.
You can choose to invest in an area, and pay either $500, $1000, or $2000 to upgrade that area. Depending on the warrior's Wisdom scores and how much money you invest, the place will gain more "exp".
Any given location can go up to level 3, and no higher. On average, if you send in 6 warriors with an average combined Wisdom of...about 65 or so, and pay $2000, that location will go up by 1 full level.
An added perk is that, the more you invest in places, the more locations will appear. All kingdoms have a maximum of 6 places that will appear, including the Bank, and they can all be leveled up in the same way (except the Bank, which never has a level).
What I'd recommend is to max out your Gold Mines first, then that way you can harvest more money to upgrade your other locations. (A level 3 Gold Mine also has a pretty good chance of making Meowth appear, who increases mined gold by 50%, so you will rack up money really fast this way.)
Just keep in mind that when you start a new story episode, everything gets reset. Hope this helps!