The roundel is 28cm in diameter and cast in bronze by the lost wax process. The image of the 'Virgin and Child' carved in relief in a roundel was rare before the Renaissance but in the 15th century it became a standard element in the upper part of wall monuments.
The circular design corresponds to Renaissance architectural forms based on pure, geometric shapes. In emphasizing architectural features in his roundel, instead of treating it simply as a frame, Donatello creates the effect that we view the Virgin and Child through an aperture in a thick wall and behind a railing.
The crescent shape of the reveal above, combined with the bulge of the railing below, define a shallow stage for the scene and suggest a low viewing point. This is a dramatic advance over earlier roundels of the Virgin and Child on the same small scale, where the space is not defined. The architectural quality of the composition was such that within a few years (about 1457) Donatello adapted it for a large roundel in Siena Cathedral, where the spectator indeed saw it from below; the illusion of seeing figures within a 'port-hole' is very convincing.