I think that the main element in Celtish warfare was the charge and ambush.If we look at the later "Celtic" nations and how they fought the charge was everything.Like the Scottish the charge gave them victory or defeat.Whether it was successfull or broken.I think ive read somewhere that celtic swordsmen tryed to roll under the pikes when facing macedonians.Once they succeeded in that the phalangites were helpless.But if i remember right the Macedonians adapted in to that tactics very fast.If i would make a little generalisation.I think that the key on the succes in war for mediterranian people were co-operation and dicipline,when for the Northern people it was the fierce death abhoring charging to the enemy and the greater power of the single warrior.