At the rear of the church, facing the Piazza di San Martino ai Monti, is the impressive apse wall. Today there are two rectangular windows, protected by iron grilles, but in the fabric it is apparent that there used to be three round-headed windows. In the 18th century, two of these were intact and the middle one had been converted into a rectangular window that is now completely blocked. The little square window below it is used to light the crypt.