One of the most remarkable developments of modern finance was the emergence of a public debt. Governments of different kingdoms – including papal kingdoms – were able to increase their resources due to credit and, more importantly, these governments were able to make payments abroad through bankers who had previously lent the funds. These bankers lent money in order to finance armies or wars, and their loans constituted short-term debt. The development of this system in Castile began at the end of the fifteenth century, when the Catholic kings needed loans in order to conquer the Muslim city of Granada. Scholars have highlighted this event as the origin of Charles V's loans; so, the fiscal and financial system of the Habsburg dynasty had precedents before 1516.

Charles V's fiscal system took its influences from Castile and the Low Countries. The administration was built in a previous period (the Contadurías of Castile) and the reforms of 1523–5 were influenced by the model of Flanders. But, if there is an area in which tradition and innovation went together, it was debt. Castile had several types of debt and credit systems in place dating from the fourteenth and fifteenth centuries, while the Catholic kings continued with a new type of juros with more modern elements. Why did public credit arrive? Traditionally, scholars have always recognized that there was the need to increase expenses to build a new model of army, and this would include a process of authority centralization and fiscal increases. In this paper, we argue about the importance of financial markets in attracting more resources to Castile in the first decades of the sixteenth century. The increase of expenditure was very important, of course; this increase, however, would not have been possible without a new context. Money, credit, merchant networks and, in general, a very good economic situation characterized this context. Moreover, production growth and merchant profits were a reliable source of potential income for the monarchy. We cannot forget that the fiscal system was a way to earn money as well.