From what I've read, Ptolemy Keraunos hastily raised a relatively undersized and undertrained force in his arrogance. Macedon had not really faced any threats from "barbarians" since Phillip or Alexander and he underestimated the power of the Gauls. Don't know if there are any accounts of the battle but he did march north to meet the Gauls. It wouldn't surprise me if the terrain was ill suited to the phalanx as well.
Bookmarks