This drawing is of particular interest since it shows an artist furnishing the design for his painted altarpiece, along with a scheme for the position of the altar it was intended to decorate — the top of which is indicated by the Maltese cross seen in perspective — and its architectural frame. The altarpiece, which is capped by a broken pediment, is set within a tablet that breaks forward from the Ionic pilasters and cornice to which it attached. At the top of the structure, in the space left at the center of the broken pediment, is a figurine of the Infant Christ as Salvator Mundi: he holds the globe at his side with his left hand and raises his right in blessing. Castello presumably submitted the drawing to his ecclesiastical patrons for their approval and his presentation of the whole scheme almost certainly indicates that he would have had charge over the construction of the entire monument, not just the painting of the sacra conversazione composition that forms its decorative focus. The utilitarian purpose of the study explains the unadorned, schematic quality of the drawing. Unfortunately, it is not clear whether this particular altar was destined for a private chapel within a church or whether it was to be merely positioned against the nave wall of such a church. The presence of St. Benedict standing on the right within the altarpiece design would appear to indicate that the altar decoration was destined for a monastery church of the Benedictine Order, the oldest Western monastic Order.

Turner, Nicholas, From Michelangelo to Annibale Carracci. A century of Italian drawings from the Prado, Chicago, Art Services International, 2008, p.166,288