The kings of Castile maintained a personal cavalry guard through much of the fifteenth century, consisting of practicing Muslims and converts to Christianity. This privileged Muslim elite provides an interesting case-study to propose new theories about voluntary conversion from Christianity to Islam in the Iberian Peninsula, as well as the ways of assimilation of such a group into the local and courtly environments where they lived thereafter. Other subjects involved are the transformation of royal armies from feudal companies to regimented, professional forces including a well-trained cavalry, which in Castile was formed partly by these knights. Their descendants had to endure the changing policies conveyed by Isabel and Fernando, which increased discriminatory habits towards converts in Castilian society.