He accordingly concentrated his forces more and more, strengthened himself by every means in his power, and advanced further and further into the interior of the country. Antigonus began to retire, desirous, perhaps, of reaching some ground where he could post himself advantageously. Pyrrhus, acting with his customary energy, soon overtook the enemy. He came up with the rear of Antigonus's army in a narrow defile among the mountains; at least, the place is designated as a narrow defile by the ancient historian who narrates these events, though, from the number of men that were engaged in the action which ensued, as well as from the nature of the action itself, as a historian describes it, it would seem that there must have been a considerable breadth of level ground in the bottom of the gorge. The main body of Antigonus's troops was the phalanx. The Macedonian phalanx is considered one of the most extraordinary military contrivances of ancient times. The invention of it was ascribed to Philip, the father of Alexander the Great, though it is probable that it was only improved and perfected, and brought into general use, but not really originated by him. The single phalanx was formed of a body of about four thousand men. These men were arranged in a compact form, the whole body consisting of sixteen ranks, and each rank of two hundred and fifty-six men. These men wore each a short sword, to be used in cases of emergency, and were defended by large shields. The main peculiarity, however, of their armor, and the one on which the principal power of the phalanx depended as a military body, was in the immensely long spears which they carried. These spears were generally twenty-one, and sometimes twenty-four feet long. The handles were slender, though strong, and the points were tipped with steel. The spears were not intended to be thrown, but to be held firmly in the hands, and pointed toward the enemy; and they were so long, and the ranks of the men were so close together, that the spears of the fifth rank projected several feet before the men who stood in the front rank. Thus each man in the front rank had five steel-pointed spears projecting to different distances before him, while the men who stood in ranks further behind rested their spears upon the shoulders of those who were before them, so as to elevate the points into the air.