That fucking Lorica Segmentata. Regular mail was much more common for a much longer period of time, just easier to produce and maintain. From what I remember reading the segmented steel plates armor was developped specifically to counter the falx, some thracian weapon that regularly dismembered legionnaries at the shoulder, but i'm pretty sure that was from Europa Barbarorum, no idea how accurate that is.
And yeah celtic culture was almost entirely subsumed into roman one, but traces of it mostly showed up in the army. Romans more or less fought in the exact same manner as greek cities in their earlier years, citizens as hoplites and the nobility as cavalry. Only after campaigns against the samnians and later celtic tribes in the Po valley and on the iberian peninsula did they addopt the standard "legionnary" equip: Montefortino helmet, mail armor, gladius hispanensis, much larger shields with a boss in the center and javelins, all inspired by celtic analogues (although the pilum's deformation after impact, supposed to make a shield useless, was a uniquely roman addition afaik). Along with coins and other intricate metalwork, the presence of oppidae, towns, roads and extensive trade networks, it's clear the celts weren't as primitive and barbaric as roman sources painted them. Although at the time of roman expansion celts were present from the british isles all the way Anatolia, overarching generalizations don't work either way, when there could be massive demographic, geographic and cultural differences from one celtic tribe to another. Certain were know for their farmers, or traders, or mystics, or sailors, or warriors, etc.
