ABSTRACT. The counsellors in António Ferreira's tragedy, Castro, have often been regarded as scheming and corrupt. This article argues, however, that their arguments echo debates around the virtues of severity and clemency found in Seneca's De clementia and in many Renaissance treatises on kingship. Considering how two different, but equally valid, notions of what a ruler should do push against each other in the stichomythic battles of the play, I show that key terms of moral evaluation end up in flux, thereby rendering the ethical decisions in Castro much more fraught. A renewed focus on rhetoric and on Renaissance treatises on kingship also opens up a new approach to the character Prince Pedro, whose angry and uncontrolled speeches might have led early modern audiences to consider him the most wayward in the play. KEYWORDS. António Ferreira, Castro, Portuguese drama, tragedy, clemency, rhetoric, ethics, Seneca, De clementia.