At the beginning of the modern age, the Castilian financial markets emerged with great dynamism. The celebration of important fairs in Medina del Campo, Medina de Rioseco, or Villalón generated an ideal environment for the meeting of merchants, moneychangers, wool producers, and many other local and foreign buyers and sellers. In this context, the growing number of commercial operations led to an important demand for credit which was mostly channeled through the moneychangers. The moneychangers of Medina del Campo, Valladolid, Burgos, or Segovia offered their services as moneylenders and financial intermediaries to support a large part of the financial framework of the fairs. This paper reconstructs the financial networks generated by the moneychangers and studies their intermediation work between merchants and communities, based on the contracts signed in the notary offices. We observe the importance of links such as neighborhood or family in creating or strengthening networks or the influence of the mercantile and financial strategy of the mercantile communities that attended the Castilian fairs in generating networks. All of this is based on the foundations of the SNA as a support methodology.