Complex sentence: At the time, he did not know what this meant, but when he came upon the Bosporus he understood: on the opposite eastern shore was a Greek city, Chalcedon, whose founders were said to have overlooked the superior location only away.
Split into simpler sentences: At the time, he did not know what this meant. But when he came upon the Bosporus he understood: on the opposite eastern shore was a Greek city, Chalcedon, whose founders were said to have overlooked the superior location only away.