In the middle of the Isthmus of Corinth stood the city, and in the midst was a fort called Acro-Corinthus, perched on a high hill in the very centre of the city, so that whoever held it was master of all to the south, and old Philip of Macedon used to call it the Corinthian shackles of Greece.  The king of Macedon, Antigonus III., now held it; but Aratus devised a scheme to take it.  A Corinthian named Erginus had come to Sicyon on business, and there met a friend of Aratus, to whom he chanced to mention that there was a narrow path leading up to the Acro-Corinthus at a place where the wall was low.  Aratus heard of this, and promised Erginus sixty talents if he would guide him to the spot; but as he had not the money, he placed all his gold and silver plate and his wife’s jewels in pledge for the amount.