A good start is to avoid the Bronze Age Collapse of the Kingdom of Hatti and likewise, avoid what seems to be some sort turmoil in Northern Europe in the Late Bronze Age. The Tollense Battle Site, a drop in estimated population across Europe and so forth, point in my view that some sort of collapse was occurring in Europe during the Later Late Bronze Age. I suspect, that the Iron Age Celtic peoples of Europe, were the heirs of this collapse.
Regardless, my opinion is that these peoples constituted a fairly advanced culture. Sources would suggest that the Celtic peoples of all the lands they resided constituted a nobility, a sort of caste system, a complex worldview, knowledge of writing (yet unwilling to use it in many areas; some Celtic peoples used a Celtic version of Aramaic), kings, local production of jewelry, extensive and widespread mining and a series of customs that the Greeks found honorable. I take the Greek historians and authors' point seriously, that to the Greeks of the Classical era, the Celts resembled their Bronze Age predecessors. Epic in style, battle and customs, aristocratic, bold, overly flamboyant and yet a bit savage, averse to writing and not progressive. If I am not mistaken, many of the Greek writers especially later, found the Celts to be icons, whose men were of ideal shape, appearance and so forth and hence, wished the Greeks to be them. That they had lost their ancient vigor of the Bronze Age, and they needed to re-learn this magnificence from the Celts or the Scythians.
Therefore, understanding this, I would say that the Celts were advanced, just as so as the Greeks. Yet their advanced manners were more in the mold of Bronze Age Europe, different from the governmental and ideological changes of the Aegean. Their societies were conquered by the Roman Republic, not due to Rome having some technological or superior advantage, but due to its logistical strength, the success of various generals, large conscription pool and an expansion driven policy by Rome that the Celtic peoples were seemingly less atune to.
