Cathy made contact with an expat American artist living in Istanbul.  She gave Cathy the directions to this underground Byzantine ruin (palatial) that is private ownership and apparently private restoration.  The artist, Trici Venola, includes a sketch of this place in her book.  Damp and musty, no different than Roman sites we visited in the past.   Except we were alone, there was no cost to get in, and the light timer went off leaving us temporarily in the dark until we triggered a sensor.