The sign by the wall is hard to read at the scale of this blog photo, but it says this: " eski mezarliga defin islemi yasaktir", which translates to: "treasure hunting in the graveyard is prohibited". The practice of digging up old grave sites (like this one at the monastery) for ancient treasures was not uncommon in years gone by. It's been stopped now...though at this point there's not likely much left to steal anyway.