The original city of Seleucus was laid out in imitation of the grid plan of Alexandria by the architect Xenarius. Libanius describes the first building and arrangement of this city (i. p. 300. 17). The citadel was on Mount Silpius and the city lay mainly on the low ground to the north, fringing the river. Two great colonnaded streets intersected in the centre. Shortly afterwards a second quarter was laid out, probably on the east and by Antiochus I Soter, which, from an expression of Strabo, appears to have been the native, as contrasted with the Greek, town. It was enclosed by a wall of its own.