Previous to this the greatest dome was that of the Pantheon at Rome, but this was carried on an immense wall 20 feet thick, and with the exception of small niches or recesses in the thickness of the wall could not be extended, so that Justinian apparently instructed his architect to provide an immense hemicycle or apse at the eastern end, a similar apse at the western end, and great arches on either side, the walls under which would be pierced with windows.