The temple compound was surrounded by a portico: a raised, covered walkway whose roof rested on an open row of columns. Porticos were common in Roman architecture, since they offered shelter from rain and sun.  However, this feature was especially important for temples of Isis, since their worship services were very often contained processions. Such a procession would have looked quite striking when glimpsed through the columns of the portico and the areas of light and shadow in the Temple.  The importance of the processions is highlighted by the fact that an image of a priest in the procession was painted in the center of each wall panel.  In effect, the viewer would see the real procession and the one painted on the wall at the same time, mirroing each other.