In Rome, on Via XX Settembre, stands the Catholic Church of Santa Susanna. While the current structure dates back to 1475, its origins are much older, having undergone numerous restorations and reconstructions over time. One of the main figures behind these reconstructions was Pope Leo III (795-816), who not only rebuilt the foundations but also commissioned a beautiful apse mosaic.

Unfortunately, this mosaic was destroyed in 1595, though a few sketches exist that reproduce its imagery. Regrettably, these reproductions focus primarily on two specific figures—Leo III and Charlemagne—leaving the remaining characters overlooked. However, through some textual descriptions, the overall composition can still be envisioned. At the center was Christ, with the Madonna, St. Peter, St. Susanna, and Leo III to His left, while St. Paul, St. Gabinus, St. Caius, and Charlemagne were positioned to His right.

The mosaic reproductions reaffirm Leo III’s role in the church's re-establishment, as he is depicted in traditional papal attire holding a miniature replica of Santa Susanna.

What draws particular attention is that this apse mosaic deviates from the standard iconography of the era, which typically featured only seven figures; in this layout, the additional figures would be Charlemagne and the Madonna. Leo III held a special veneration for the Madonna.

Charlemagne’s presence is explained by his role as a sponsor—perhaps unintentionally—of the church’s reconstruction. The work was partially funded by an important donation made by the Frankish king to the pope, known as the Avar Treasure, acquired in 796 following his conquest of their ring fortress.