This essay deals with the negative consequences brought by the expansion of the consolidated debt since the XVIth century through the massive issues of long term annuities (juros) on the Castilian economy. The article analyses the reasons which reduced the appeal of these financial assets among private investors. This prompted the Royal Treasury to follow an alternative course and, trying to expand its credit base, the Crown resorted to the financial systems of the Castilian cities, which became closely subordinated to the needs of the Royal Treasury.