The Underground Cistern was built by the Romans to provide a steady water supply to Istanbul if the city were attacked. When used, water filled the cistern to the top of the columns. The materials used were brought from pagan structures in conquered lands. With Christianity on the rise the pagan head structures of Medusa, which once stood at the entrances of buildings as guardians, were placed upside down and in the back corner of the Cistern. An idea of how they felt about the pagans.