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Recent researches show that part of Alexander's wall can be traced back to 4th century Bc and even earlier, of course the major part of this wall has been built and rebuilt in Parthian and Sassanid periods.This wall wall which is the largest defensive wall in the world after the Great Wall of China, starts from the Caspian coast, circles north of Gonbade Kavous, continues towards northwest and vanishes behind Pishkamar Mountains.
At parts, this wall is 6 m wide and in other parts the width is 10 m, which proves that the thickness of the wall differs in various regions, according to the natural features and soil configuration.
Castles have been built at different distances. The longest distance between castles is 50 km and the shortest is 10 km.The 40 identified castles vary in dimension and shape but the majority are square fortresses.

"Recent researches show that part of Alexander's wall can be traced back to 4th century Bc and even earlier"Hi Cyrus, This is really great news. Thanks! Do you have a reference? (If it is written in Farsi, don't hesitate; I can not read it, but I have a friend who can and will gladly translate it for pothos.)Jona

Perhaps that was part of his defence line against the Skythians. Probably designed to hold the frontier after Spitamenes was defeated (?)

I read it in an Iranian magazine (Tarikh va Jaqrafiz), of course it doesn't say that it was built by Alexander but the Achaemenids.

Thanks!Jona