The aim of this paper is to shed new light on a central, yet much misunderstood source from the initial stage in the process of the codification of Danish law. On the literal level, the Lex castrensis, written in the 1180s, represents a description of changes in the internal jurisdiction of the royal court from the time of the reign of Cnut the Great to the author's present. In Danish as well as international scholarship this deceptively simple text has frequently been treated either as a ‘law code' or ‘law book' in itself or as a reflection of actual legal practice. Yet here I will contend that the Lex castrensis is in fact a political-legal treatise masked as pseudo-history with an explicit didactic purpose as a ‘schoolbook' for future administrators. As a learned ideological construct with a remarkable afterlife, Sven's work may be regarded as part of the intellectual preparation for future legislative processes.