The rectangular portico is 5th century. It originally had an enormous portal divided into three rectangles by a pair of monumental porphyry Composite columns. The screen walls and large entrance doorcase inserted into the portico are originally 12th century. Affixed to the wall just to the left of the portal are the remnants of a ribbed Corinthian pilaster in white marble, supporting the end of the entablature. This is the sole survivor of the 5th century cladding of the baptistery and portico.