Central apse shows Christ on a mountain from which spring four rivers of paradise. Blessing with right hand and holds book in left.  Purple garments.  Five apostles on each side, one carries a long cross, none of the others have attributes.  All figures nimbed.  Over Christ’s head a cloud with lightning.  Border of mosaic consists of flowers which spring from two vessels.  In the centre of border, over Christ’s head is the monogram of Leo III:
                                               L
                                            PEA
                                              O

A second border encircles the arch, containing inscription: GLORY TO GOD IN THE HIGHEST: AND ON EARTH PEACE TO MEN OF GOODWILL.  At the feet of the figures runs a frieze with an inscription: GOING THEREFORE, TEACH YE ALL NATIONS, BAPTISING THEM IN THE NAME OF THE FATHER AND THE SON AND THE HOLY GHOST.  AND BEHOLD I AM WITH YOU ALL DAYS, EVEN TO THE CONSUMMATION OF THE WORLD.  On the triangular wall left of the apse is a picture of St. Peter, Leo III, black haired, with rectangular nimbus, and Charlemagne receiving a standard from St Peter, six red roses in a blue field.  Charlemagne has two twisted moustaches.

In the vault of the apse are ten apostles, and the eleventh is in the triangle of the apse wall.