I am wondering what is downstream of the Keep. There are deep hardwood forests in close proximity to a river, which means that the real wealth of that area is in timber industry. The hardwood logs could be floated down the river to a larger town where they can be milled into lumber and sold. Player characters can negotiate a percentage of the profits as stockholders as payment for the service of clearing out of the humanoids from the caves of chaos which threaten the Keep.
As the wild forests are harvested, fruit orchards and vinyards can be planted. Oak trees will be needed for barrel making, Wine and Cider can be floated downriver in keelboats to a larger town or city.