Focus on this artwork and explain the New ideas on attribution.
The debate took on a new dimension a few years later when  a previously unknown fresco was discovered on the western wall of the Sala del Mappamondo. It is probably one of the castle frescoes that disappeared behind plaster circa 1345 when the Mappamondo was installed in the hall. It is partly overlapped by the Guidoriccio, has a similar theme (two persons and a castle) and is of high quality. Some point to Duccio as the creator, others argue that this underlying fresco is definitely by Simone Martini. According to Moran and Mallory, this is Martini's fresco of the castle in Arcidosso. This would mean that the Guidoriccio cannot be the Montemassi fresco painted by Simone Martini in 1330 because it partly overlaps the image of Arcidosso, which was painted a year later in 1331.
The main issue in the Guidoriccio debate therefore no longer was whether perhaps the equestrian image had not been painted by Simone Martini, but whether the entire fresco could be attributed to him. It has been suggested that the Guidoriccio was painted by Lippo Memmi, a brother-in-law of Simone Martini who had a similar style. However, this theory is not widely supported.
Lippo Memmi Duccio Simone Martini Arcidosso Montemassi fresco