This medieval chapel rests against the left side wall of the Temple. The building, dedicated to Saint Césaire, is composed of a small square nave and a vaulted apse in cul-de-four (quarter sphere vault). The floor of the chapel is located at 0.6 m below the level of the temple's pedestal. Traces of a staircase leading down to the chapel from the temple were discovered as well as an opening in the lateral wall of the temple. This opening was originally framed by two small columns, of which only one remains. The preserved column is an exact replica of the great column of the Temple. The capital appears to be an addition, since it is wider than the small column. Some conserved aspects of the medieval church show a close stylistic similarity with the ancient monument, notably the flutes of the column. The chapel was consecrated under the double name of Saint Césaire and Saint Pierre by the Archbishop of Arles, accompanied by the Bishop of Apt, in the middle of the 11th century, in 1054.