I can explain this I think. in Table 8.1 of my book on The Quest for the Tomb of Alexander the Great I give a list of all the known ancient sources on the size of ancient Alexandria. There are three that give the perimeter of her walls: Curtius at 80 stades, Pliny at 15 miles and Stephanus Byzantinus at 110 stades. All of these are significantly different to the 15.84km given by Katerina Peristeri. However, at the end of my Table I give the perimeter as calculated by Mahmoud Bey on the basis of excavations in 1865. Mahmoud gives the perimeter as 96 Alexandrian stades and he gives an Alexandrian stade as 165m, hence 15,840m. So that is where the lady got her measurement from. However, having read Mahmoud Bey's book, I can also tell you that he largely guessed the exact line of the walls in some areas, so it is quite dubious whether Katerina Peristeri's value is very accurate. Also it is doubtful whether that outer wall was part of Deinocrates' original plan. It is essentially the wall line of the city at its zenith around the time of Augustus. As I have mentioned the only fragment surviving now of early Ptolemaic wall is in the line of a much smaller circuit, near the middle of Mahmoud Bey's city and encompassing its central crossroads. As I have said, that is a better candidate for Deinocrates' handiwork (against this Curtius says that an 80 stades circumference was planned by Alexander: octaginta stadiorum muris ambitum destinat, but this probably came from Cleitarchus who lived in the city in the mid 3rd century BC, when the city probably did have an 80 stade circumference). On the whole I think this thing is likely to be a random coincidence additionally because Katerina Peristeri is comparing a diameter with a perimeter, which is a bit odd.
Best wishes,
Andrew
