3. This is the only road between Thessaly and the northern districts, which passes in its whole length along a valley; all the others are mountain-passes. Such was the other road to Macedonia, which crossed mount Olympus (ἐσβολὴ Ὀλυμπική).(69) This road, too, begins at the strongly-fortified city of Gonnus, the key of the country towards the north; and it then goes along the southern side of Olympus, till it reaches the cities of Azorum and Doliche. Between these two towns is a place where three ways met.(70) The chief road passes in a northerly direction over the summit of the Cambunian chain to the Macedonian highlands; and it was here that Xerxes set fire to the woods in order to open a passage for his army, which the Greeks had expected along the more practicable way through Pieria and the valley of Tempe; and it was often in the Roman wars traversed by large armies.(71) From the south of Olympus two difficult mountain roads led over the heights of Olympus, connecting Northern Thessaly with Pieria. The one avoided the valley of Tempe, as it passed by the fortress of Lapathus to the north of that defile,(72) then along the small lake of Ascurias, whence there was a view of the town of Dium on the sea-coast, at the distance of 96 stadia; after which it descended into the plains of Pieria. We should, however, more particularly notice the other road, taking a more northern direction, and passing over the lofty sides of Olympus, where formerly there stood the castle of Petra, and the temple of the Pythian Apollo, commonly called Pythium, together with a village of the same name,(73) the height of which Xenagoras, by a geometrical measurement, ascertained to be 6096 Grecian feet.(74) From this point there was a mountain-pass leading down to the coast to Heracleum and Phila in Pieria, and another way led along the ridge of Olympus by difficult and dangerous passages, as far as Upper Macedonia.(75) These mountain-passes and defiles have not been explored by any modern traveller; but it was important for our subject to discover their position from the writings of the ancients. Not only did Perseus and Æmilius Paulus here contend for the fate of Macedonia, but it was in this region that the Greek nations of the heroic age disputed the possession of the fertile Thessaly. There was once a time when through these passes the nations pressed down, to whose lot the finest parts of Greece were once to fall; here every step was gained with labour, while the sons of the mountain inured themselves to hardships in their incessant wars. Of the numerous citadels which in these districts cover every important point, the greater number were probably built at a very early period. Thus there were three fortresses(76) to defend the pass of Olympus, or the road from Gonnus to Azorum and Doliche, which two places, together with Pythium on the mountain, were comprehended under the name of the Pelagonian Tripolis.(77)