Originally this was linked to the open space - the ambulatory - by means of three great arches and two intermediate apertures, but is today closed by the huge altar piece depicting "The Glory of SS. Charles and Ambrose", painted by Carlo Maratta on the very spot between the years 1685 and 1690.

The high altar with its bronze decorations was sponsored by Cardinal Scotti and completed in 1725. The convex of the apse is decorated with a false tapestry depicting "St. Charles among the Plague-stricken", and the vault showing, "St. Charles in Glory" both executed by Brandi between 1675 and 1677.