The good state of preservation of these buildings and their frescoes was due to their location, close to the main city wall facing west, and the military necessity to strengthen the wall. The Sassanid Persians had become adept at tunneling under such walls in order to undermine them and create breaches. As a countermeasure the Roman garrison decided to sacrifice the street and the buildings along the wall by filling them with rubble to bolster the wall in case of a Persian mining operation, so the Christian chapel, the synagogue, the Mithraeum and many other buildings were entombed. They also buttressed the walls from the outside with an earthen mound forming a glacis and sealed it with a casing of mud brick to prevent erosion.