Some examples from earlier postings; the use of peasantry for possible defense or rebellion, the Guilds supporting a strong militia presence within a kingdom, and farming that will colour to the table top and mean revenues (prestige points) for the kingdom.
Money lending has a long history, but the medieval period saw the development of features of banking that set the foundation to many aspects of current financial transaction.
Kingdoms necessitating the use of arms and could not raise sufficient taxes, were forced to take loans to raise armies. For the lending house this did bring high risks as some bankers discovered when Edward III defaulted on his loans. Those houses that survived such ventures did so with by utilizing a large network of agents with an intimate knowledge of their respective regions.
Throughout Europe and Asia Minor, kingdoms minted their own coin. With no regulation as to size and content, Banks or money changers had to set exchange rates in order to accept coinage from other lands. This meant coin could be over or undervalued leading to severe consequences for the money lender.
With respect to the Empire, the most common coin are the thaler, florin and mark. It would take another century before the establishment of an Imperial Minting Ordnance which would set value to diverse silver and gold coinage used within the Empire.
Application to the DBA campaign
The basic system uses prestige points to function as a currency to rebuilding armies. Prestige points are gained through territorial conquest and/or victory in the field. Elimination from the campaign meant total loss of the army or seizure of the player’s capital.
A few things I have considered to help ease the waning fortunes of players are the possibility of loans (prestige points) to facilitate the hiring of mercenaries, or to use as bribes to bring a neutral kingdom to your side, or bribes to deter an enemy bent on invasion.
To simplify the system, players would announce their need for a loan and roll a die to determine the "commission or insurance" that must be added to the loan. The time nequired to repay the loan could also be worked into the same table. Those players who manage to repay their loans have the satisfaction of having done so, while defaulters move to another stage of the campaign.
Historically, unions or marriages were done to solve such problems. For the shrewd player this would mean acquisition of territory without the use of force. I have not worked out how this should be done, but it would certainly offer a resourceful player an interesting option. The defaulter could continue as a vassal, maintaining garrisons and supplying "allied" contingents for the field or start afresh. Adding more kingdoms will certainly prompt me to add more armies. What is there not to like about this.