In the turn of the fifteenth century, a republican vocabulary began to figure prominently in different texts and documents produced in the Crown of Castile. Terms such as "common good" and "good government", previously absent from Castilian records, were now becoming central for making different kinds of political claims. In what circumstances this discursive change took place? How and to what end it affected social fabrics of power? This project explores the relations between republican discourses, social conflicts and civic practices in late medieval Castilian cities. Drawing on a wide range of archival and literary sources, I examine how different social actors appropriated and deployed ideas and claims concerning "good government" and "common good" to engage with a complex political world. My research is particularly concerned with the ways in which republican rhetoric and imagination were deployed in five domains: negotiations over royal authority; propaganda of different urban groups; violence against Jews and conversos including the emergence of notions of "purity of blood"; the production of public spaces in Castilian cities; and the eruption of urban popular protests including the attempts made by Castilian authorities to repress them. In all these areas, I argue, republican thought was "put to work" in meaningful and complex ways which have been largely overlooked. This project proposes the study of the historical negotiations of republican concepts as a perspective from which to rethink these domains and the connections between them.