Complex sentence: First was the Gallic version of the phalanx like the Hellenistic phalanx it was several lines of men with their spears lowered to form a thicket of spears.
Split into simpler sentences: First was the Gallic version of the phalanx. Like the Hellenistic phalanx it was several lines of men with their spears lowered to form a thicket of spears.