Complex sentence: Their job was guard the flanks of the large and unwieldy pike phalanx, an armored soldier with a 18-22 ft. Pike is not particularly agile or able to turn quickly, so they would prevent attacks on the very vulnerable sides of the formation.
Split into simpler sentences: Their job was guard the flanks of the large and unwieldy pike phalanx, an armored soldier with a 18-22 ft. pike (weapon). Pikemen are not particularly agile or able to turn quickly, so hypaspists would prevent attacks on the very vulnerable sides of the formation.