What should a landed PC spend money on?

Income comes from tax. Tax comes from earnings. Earnings come from either a) high production or b) profitable trade.

The simple answer is infrastructure. Roads, for instance, allow merchants to travel to and from commerce centres easily. They allow farmers to rapidly transport their harvest to market. They allow local militia to quickly respond to bandit threats.

Security is another important thing. You want local constabulary, magistrates, etc to deal not just with external threats but internal disputes ("he bought this but never paid", etc). Without that people lose faith and crime takes over.

Consolidation is good for making money, but not for keeping people happy. 20 peasant families with 5 acres each will make a lot less money than 5 peasant families with 20 acres each. Of course, that leaves 15 peasant families living in squalor in the town, with both parents working as hired field hands on the richer peasant farms to earn enough for mouldy bread and watery stew.

Exploitation of natural resources is another option. Mines, for instance, make great money, but also require large upfront investments and a long time before they start producing. There's also a lot of workers in a small space, so it will add to crowding in local towns, etc.

Think about land use. Is the land all farmland? If not, what percent is forest and able to be harvested for lumber, or hunted for game? Is it worth clearing all of the forests for more farmland? Are there lakes or rivers or coastlines for fishing? Are their specialised sources of certain spices, textiles, or whatever around? How would players maximise not only their production, but also their demand? If a neighbouring barony begins its own silk weaving industry, well perhaps a little bit of sabotage might help to retain your monopoly and maintain your profits.

/r/DnD Thread