Plasters and mortars from Lardirago Castle (Pavia, northern Italy) have been characterized by means of petrographical and chemical analyses in order to achieve technical and historical evidence of material production. The study focused on the oldest nucleus of the structure, where hidden plastered masonry has been discovered recently. Textural and compositional data of the binder and the aggregate fractions were collected by means of optical microscopy and electron microscopy coupled with microanalysis. These data permit the identification of building materials with similar technical and compositional characteristics, which can be related to different building phases.