EXTERIOR
The whole building stands on a 1.3 m high base which originally extended a further 7 metres in front of the colonnade. Steps in Numidian yellow marble extended from the outer ends of this base. The building consists of two principal parts - the porch, which is very Classical Greek in presentation, and the circular main building which is much more Roman in style and reminiscent of the architecture of the large Roman baths. The circular building is built with brick and concrete but was originally faced with white marble stucco to match the porch in appearance. The dome is concrete with the external surface originally covered in sheets of bronze but these were removed by Constans II in 663 CE.
The porch measures 33.1 x 13.6 m and presents a front colonnade of eight Corinthian columns 11.8 m high. The monolithic column shafts are in Mons Claudianus and Aswan grey granite with the bases and capitals in white Pentelic marble. The pediment above the columns is now empty but drill holes suggest there was originally an emblem of some sort, possibly an eagle or wreath which would have been in gilded bronze and symbolised Jupiter. The porch was faced with white Pentelic marble and is decorated with reliefs showing objects used in religious sacrifices (such as dishes, boxes and axes), garlands and candelabras. The interior of the porch measures 34 x 20 m and has four rose-pink columns creating three aisles. The pavement is restored but reflects the original design with grey granite rectangles and circles in white marble. The interior of the porch was also panelled with marble but this has since been lost, revealing the brickwork.